Make DMChannels and GroupChannels mentionable

[PythonCoderAS]

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Summary

This makes DMChannels and GroupChannels mentionable. These do properly show up when mentioned.

Checklist

• [ ] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [ ] This PR fixes an issue.
• [x] 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, ...)

[justanotherbyte]

These are just message links that appear as mentions in the discord client. DMChannels and GroupChannels can't be mentioned.

[PythonCoderAS]

They are not message links, they seem like one, but try to type in <#GroupDMChannelID> in your discord and send the message

[trevorflahardy]

These are just message links that appear as mentions in the discord client. DMChannels and GroupChannels can't be mentioned.

The keyword here is "Copy Channel ID", it can be mentioned using said channel ID now.

DM Channel: Group Channel: