Clarify Message.nonce documentation

Fix #2451

discord/message.py

@@ -406,7 +406,7 @@ class Message(Hashable):
         The actual contents of the message.
     nonce
         The value used by the discord guild and the client to verify that the message is successfully sent.
-        This is typically non-important.
+        This is not stored long term within Discord's servers and is only used ephemerally.
     embeds: List[:class:`Embed`]
         A list of embeds the message has.
     channel: Union[:class:`abc.Messageable`]