Clarifications to the admin api documentation (#7647)

* Clarify how to authenticate
* path params are not the same thing as query params
* Fix documentation for `/_synapse/admin/v2/users/<user_id>`

docs/admin_api/delete_group.md

@@ -4,11 +4,11 @@ This API lets a server admin delete a local group. Doing so will kick all
 users out of the group so that their clients will correctly handle the group
 being deleted.
 
-
 The API is:
 
 ```
 POST /_synapse/admin/v1/delete_group/<group_id>
 ```
 
-including an `access_token` of a server admin.
+To use it, you will need to authenticate by providing an `access_token` for a
+server admin: see [README.rst](README.rst).