Add warnings about MSC3861 on certain APIs. (#16168)

changelog.d/16168.doc

+Document which admin APIs are disabled when experimental [MSC3861](https://github.com/matrix-org/matrix-spec-proposals/pull/3861) support is enabled.

docs/admin_api/account_validity.md

 # Account validity API
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 This API allows a server administrator to manage the validity of an account. To
 use it, you must enable the account validity feature (under
 `account_validity`) in Synapse's configuration.

docs/admin_api/register_api.md

 # Shared-Secret Registration
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 This API allows for the creation of users in an administrative and
 non-interactive way. This is generally used for bootstrapping a Synapse
 instance with administrator accounts.

docs/admin_api/user_admin_api.md

@@ -218,7 +218,7 @@ The following parameters should be set in the URL:
 - `name` - Is optional and filters to only return users with user ID localparts
   **or** displaynames that contain this value.
 - `guests` - string representing a bool - Is optional and if `false` will **exclude** guest users.
-  Defaults to `true` to include guest users.
+  Defaults to `true` to include guest users. This parameter is not supported when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
 - `admins` - Optional flag to filter admins. If `true`, only admins are queried. If `false`, admins are excluded from 
   the query. When the flag is absent (the default), **both** admins and non-admins are included in the search results.
 - `deactivated` - string representing a bool - Is optional and if `true` will **include** deactivated users.
@@ -390,6 +390,8 @@ The following actions are **NOT** performed. The list may be incomplete.
 
 ## Reset password
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 Changes the password of another user. This will automatically log the user out of all their devices.
 
 The api is:
@@ -413,6 +415,8 @@ The parameter `logout_devices` is optional and defaults to `true`.
 
 ## Get whether a user is a server administrator or not
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 The api is:
 
 ```
@@ -430,6 +434,8 @@ A response body like the following is returned:
 
 ## Change whether a user is a server administrator or not
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 Note that you cannot demote yourself.
 
 The api is:
@@ -723,6 +729,8 @@ delete largest/smallest or newest/oldest files first.
 
 ## Login as a user
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 Get an access token that can be used to authenticate as that user. Useful for
 when admins wish to do actions on behalf of a user.
 

docs/usage/administration/admin_api/registration_tokens.md

 # Registration Tokens
 
+**Note:** This API is disabled when MSC3861 is enabled. [See #15582](https://github.com/matrix-org/synapse/pull/15582)
+
 This API allows you to manage tokens which can be used to authenticate
 registration requests, as proposed in
 [MSC3231](https://github.com/matrix-org/matrix-doc/blob/main/proposals/3231-token-authenticated-registration.md)