Properly document that categories throw 404's on create_invite.

discord/abc.py

@@ -710,7 +710,7 @@ class GuildChannel:
     async def create_invite(self, *, reason=None, **fields):
         """|coro|
 
-        Creates an instant invite.
+        Creates an instant invite from a text or voice channel.
 
         You must have the :attr:`~Permissions.create_instant_invite` permission to
         do this.
@@ -738,6 +738,9 @@ class GuildChannel:
         ~discord.HTTPException
             Invite creation failed.
 
+        ~discord.NotFound
+            The channel that was passed is a category or an invalid channel.
+
         Returns
         --------
         :class:`~discord.Invite`