Add FAQ about guild specific app commands

[Soheab]

Summary

This PR adds a FAQ entry that shows all the ways to define a guild specific app command.

A note was added to copy_global_to as that method is commonly mis used or understood as a way to make commands guild specific. I'm not sure if there is a way to link to a specific FAQ.

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]

I think this FAQ entry is flawed.

The question is probably closer to "How do I restrict a command to a specific guild?" and the answer to that question is slightly different to the one being proposed here, though it does have some similarities such as a guild command.

I suggest rewording the sentence and answer with that in mind instead.

[Rapptz]

Suggested change

	.. note::
	You probably do not want to use this. This is mainly for development purposes.
	See other ways of declaring a guild specific command in the FAQ.

The docstring already mentions this. Repeating this adds no value.