docs: update the action how-to to explain the additionalProperties default flip

docs/howto/manage-actions.md

@@ -39,6 +39,8 @@ actions:
     additionalProperties: false
 ```
 
+You should **always** include the `additionalProperties` field, which controls whether users can provide properties that are not in the definition. The default value of the field changed from `true` in Juju 3 to `false` in Juju 4. If you don't explicitly include the field, the charm behaviour will differ depending on which Juju version the charm is deployed to.
+
 In the `src/charm.py` file of the charm, add a class that mirrors the
 configuration from `charmcraft.yaml`. This lets your static type checker and
 IDE know what Python type the parameters should be, and provides a place to do

docs/tutorial/from-zero-to-hero-write-your-first-kubernetes-charm/expose-operational-tasks-via-actions.md

@@ -36,7 +36,8 @@ actions:
       show-password:
         description: Show username and password in output information
         type: boolean
-        default: False
+        default: false
+    additionalProperties: false
 ```
 
 ## Define an action class

examples/k8s-4-action/charmcraft.yaml

@@ -43,7 +43,8 @@ actions:
       show-password:
         description: Show username and password in output information
         type: boolean
-        default: False
+        default: false
+    additionalProperties: false
 
 containers:
   demo-server:

examples/k8s-5-observe/charmcraft.yaml

@@ -69,6 +69,7 @@ actions:
         description: Show username and password in output information
         type: boolean
         default: False
+    additionalProperties: false
 
 containers:
   demo-server: