Update on_member_update in event reference docs

[argo0n]

Warning: We have a feature freeze till release

That means we won't accept new features for now. Only bug fixes.

Summary

If a user is given a timeout, this is captured by on_member_update, so it should reflect in the docs to avoid people being confused as to whether on_member_update is called when a user is given/no longer has a timeout

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

[argo0n]

I believe discord.AuditLogAction.member_update needs to be edited too, not sure how to edit that one though