docs: improve documentation

[NotSugden]

This PR adds some better documentation, corrects some invalid syntax in the JSDoc comments, and deprecates RPCClient#sendJoinRequest as the command has been removed

this PR only changes documentation

reference: this pr is apart of the chain of prs requested from #89 (comment)

[papaia]

As per JSDoc docs

[NotSugden]

Updated to requested changes, however there is just a nit - the VoiceSettings and UserVoiceSettings typedefs have properties marked as optional (for editing via any of the respective set* methods), but the same type is used as the return type - this would be incorrect as the properties will not be optional, not really sure of the best way to go about that or if it should even be addressed

[papaia]

Shouldn't Promise<undefined> be changed to Promise<void>?

[NotSugden]

Shouldn't Promise<undefined> be changed to Promise<void>?

thats what i thought, but #90 (comment)

[dscalzi]

Could throw in an index.d.ts while you're at it.

[NotSugden]

Could throw in an index.d.ts while you're at it.

see #89 (comment) snek doesn't want typings in the library itself