Add type notice for burn_on_read webhook posts. (#1500)

* Add `type` notice for `burn_on_read` webhook posts.

Updated the 'type' parameter description to clarify that it can be set to 'burn_on_read'.

* Update site/content/integrate/webhooks/incoming/_index.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: svelle

site/content/integrate/webhooks/incoming/_index.md

@@ -80,7 +80,7 @@ Incoming webhooks support more than just the `text` field. Here is a full list o
 | `icon_url`    | Overrides the profile picture the message posts with.<br/>Defaults to the URL set during webhook creation; if no icon was set during creation, the standard webhook icon ({{<compass-icon icon-webhook>}}) is displayed.<br/>The {{<newtabref title="Enable integrations to override profile picture icons" href="https://docs.mattermost.com/configure/configuration-settings.html#enable-integrations-to-override-profile-picture-icons">}} configuration setting must be enabled for the icon override to take effect.                                                                                                                                                                                                                                                                                                    | No                               |
 | `icon_emoji`  | Overrides the profile picture and `icon_url` parameter.<br/>Defaults to none and is not set during webhook creation.<br/>The expected value is an emoji name as typed in a message, either with or without colons (`:`).<br/>The {{<newtabref title="Enable integrations to override profile picture icons" href="https://docs.mattermost.com/configure/configuration-settings.html#enable-integrations-to-override-profile-picture-icons">}} configuration setting must be enabled for the override to take effect..                                                                                                                                                                                                                                                                                                        | No                               |
 | `attachments` | [Message attachments]({{<ref "/integrate/reference/message-attachments">}}) used for richer formatting options.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | If `text` is not set, yes        |
-| `type`        | Sets the post `type`, mainly for use by plugins.<br/>If not blank, must begin with "`custom_`".                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | No                               |
+| `type`        | Sets the post `type`, mainly for use by plugins.<br/>If not blank, must begin with `custom_` unless set to `burn_on_read`.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             | No                               |
 | `props`       | Sets the post `props`, a JSON property bag for storing extra or meta data on the post.<br/>Mainly used by other integrations accessing posts through the REST API.<br/>The following keys are reserved: `from_webhook`, `override_username`, `override_icon_url`, `override_icon_emoji`, `webhook_display_name`, `card`, and `attachments`.<br/>Props `card` allows for extra information (Markdown-formatted text) to be sent to Mattermost that will only be displayed in the RHS panel after a user selects the **info** icon displayed alongside the post.<br/>The **info** icon cannot be customized and is only rendered visible to the user if there is `card` data passed into the message.<br/>This property is available from Mattermost v5.14.<br/>There is currently no Mobile support for `card` functionality. | No                               |
 | `priority`    | Set the priority of the message. See [Message Priority](/integrate/reference/message-priority/)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | No                               |