Clearly define button max label length

[UP929312]

Currently, to find the max for this argument, you have to look at official discord docs (which is what I did first), try it and get an error (slow and painful), or check the source code (which I also did), this just puts it in the docs so you can see straight away.

Summary

Information

• 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, ...).

Checklist

• I have searched the open pull requests for duplicates.
• 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.