add explanations on how to actually include an access_token (#6031)

docs/admin_api/README.rst

@@ -10,3 +10,15 @@ server admin by updating the database directly, e.g.:
 ``UPDATE users SET admin = 1 WHERE name = '@foo:bar.com'``
 
 Restarting may be required for the changes to register.
+
+Using an admin access_token
+###########################
+
+Many of the API calls listed in the documentation here will require to include an admin `access_token`.
+Finding your user's `access_token` is client-dependent, but will usually be shown in the client's settings.
+
+Once you have your `access_token`, to include it in a request, the best option is to add the token to a request header:
+
+``curl --header "Authorization: Bearer <access_token>" <the_rest_of_your_API_request>``
+
+Fore more details, please refer to the complete `matrix spec documentation <https://matrix.org/docs/spec/client_server/r0.5.0#using-access-tokens>`_.