api-docs: Update delete-messages description for current permissions.

Prior to feature level 281, the only users who had permissions to
permanently delete any message in the organization were organization
administrators.

Currently, various realm and channel permission settings, which use
the user group model, determine if a user is able to permanently
delete a specific message.

Updates the main description of the endpoint to note these settings
and removes the note that the endpoint is limited to organization
administrators.

zerver/openapi/zulip.yaml

@@ -10077,12 +10077,39 @@ paths:
       description: |
         Permanently delete a message.
 
-        This API corresponds to the
-        [delete a message completely][delete-completely] feature documented in
-        the Zulip help center.
+        This API corresponds to the [delete a message completely][delete-completely]
+        feature documented in the Zulip help center.
+
+        A user must be able to access the content of a message in order to delete it.
+        See [channel permissions](/help/channel-permissions) for more information
+        about content access for channel messages. For direct messages, the user
+        must have received or sent the direct message to have content access.
+
+        See [restricting message deletion](/help/restrict-message-editing-and-deletion)
+        for documentation on when users are allowed to delete messages.
+
+        The relevant realm settings in the API that are related to the above linked
+        documentation on when users are allowed to delete messages are:
+
+        - `realm_can_delete_any_message_group`
+        - `realm_can_delete_own_message_group`
+        - `realm_can_set_delete_message_policy_group`
+        - `realm_message_content_delete_limit_seconds`
+
+        The relevant per-channel permission settings in the API that are related to the
+        above linked documentation on when users are allowed to delete messages in a
+        specific channel are:
+
+        - `can_delete_any_message_group`
+        - `can_delete_own_message_group`
+
+        More details about these realm and channel settings can be found in the
+        [`POST /register`](/api/register-queue) response.
+
+        **Changes**: Prior to Zulip 10.0 (feature level 281), only organization
+        administrators had permission to permanently delete a message.
 
         [delete-completely]: /help/delete-a-message#delete-a-message-completely
-      x-requires-administrator: true
       parameters:
         - $ref: "#/components/parameters/MessageId"
       responses: