docs: clarify Member.current_timeout expiry (#1063)

Tiny documentation change to clarify that `Member.current_timeout` also
returns `None` when the current timeout value is in the past, allowing
users to safely rely on this behavior.

disnake/member.py

@@ -695,7 +695,9 @@ def voice(self) -> Optional[VoiceState]:
 
     @property
     def current_timeout(self) -> Optional[datetime.datetime]:
-        """Optional[:class:`datetime.datetime`]: Returns the datetime when the timeout expires, if any.
+        """Optional[:class:`datetime.datetime`]: Returns the datetime when the timeout expires.
+
+        If the member is not timed out or the timeout has already expired, returns ``None``.
 
         .. versionadded:: 2.3
         """