Add a mention about how currently_active works

Reference: d69decd5c7/synapse/handlers/presence.py (L66-L68)

api/server-server/definitions/event-schemas/m.presence.yaml

@@ -62,6 +62,8 @@ allOf:
                   type: boolean
                   description: |-
                     Whether or not the user is currently using a device of theirs.
+                    For example, if the user's ``last_active_ago`` was within the
+                    last few minutes, they may be considered ``currently_active``.
                     Defaults to false.
                   example: true
               required: ['user_id', 'presence', 'last_active_ago']