Document NotFound exception raise in message deletion endpoints

discord/channel.py

@@ -272,6 +272,8 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
         Forbidden
             You do not have proper permissions to delete the messages or
             you're not using a bot account.
+        NotFound
+            If single delete, then the message was already deleted.
         HTTPException
             Deleting the messages failed.
         """

discord/message.py

@@ -740,6 +740,8 @@ class Message:
         ------
         Forbidden
             You do not have proper permissions to delete the message.
+        NotFound
+            The message was deleted already
         HTTPException
             Deleting the message failed.
         """