docs: exclude pins() from voice/stage channels

[shiftinv]

Summary

Resolves the documentation side of #1033. Voice/Stage channels having a pins() method isn't necessarily an issue, especially since the API returns an empty list instead of throwing an error, but this is still something that can be solved by a refactor in the future.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• 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, ...)