Add notes about contexts and installation_types only being available for global commands

[Soheab]

Summary

While in preview, only global commands can be made into user installable apps and I think the docs should note that on the guild(s) decorator and kwargs.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• 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, ...)

[Rapptz]

The wording is fine, but can you expand "kwarg" to "keyword argument" in the documentation.

[Rapptz]

Thanks