fix: custom document type

[jozsefdamokos]

Fixed some errors + some helpful additions in custom document type page

[Isengo1989]

Thx for the PR @jozsefdamokos 🎉

[ennasus4sun]

one more thing:
I had a look into the other existing page right now (Add Custom Document). I did not test it, but I think it is not adding a new document. Looks more like adding a custom config (in the example for delivery-notes) for a specific sales-channel 🤔 (I assume that's the same we can do in the admin already).
But ... it sounds like we create a new document type.
Maybe this can be written more clear by using the word "document config" instead document.

AND:
maybe also our real new document (DocumentRenderer) will need a new config after we added it (just an assumption)

[jozsefdamokos]

@ennasus4sun the other page (Add Custom Document) is correct, it adds a new document in the list of documents in the admin. It is the same you can do in the admin but this one is via the migration (for when your plugin should create a document on plugin install for example). The naming is a bit confusing, but the article says: "Documents in Shopware are stored in the database table document_base_config. Do not confuse it with the document table, which contains the actually generated documents"