Improve typing for messages and message update methods

[davfsa]

Summary

Thanks to @Le0Developer for bringing up the initial issue of is_webhook always being True for MessageUpdateEvent in the Discord server.

This had me investigate and opened up a whole can of worms where I found inconsistencies and bugs in code and documentation fixes.

Changes

• Fix the use of typing.Optional where undefined.UndefinedOr should have been used.
• Remove trying to acquire guild_id from the cached channel on PartialMessage
  • Instead, clearly document the issue Discord imposes by not sending the guild_id
• is_webhook can now return undefined.UNDEFINED if the information is not available
• Fix logic in is_human to account for the changes in types
• Set PartialMessage.member to undefined.UNDEFINED when Discord edit the message to display an embed/attachment

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

None