Improve error handling in PostStatusService

Improve error handling in PostStatusService

* Explicitly catch invalid post visibility, and 422 instead of 500
        - ~~Reject "limited" visibility~~
        - Accept empty visibilities, ignoring them
* Explicitly alert the user when a post is rejected for having both
  media and polls

The former currently allows the visibility property to be blank or null.
I do not know if this is more desirable than, for example, rejecting
empty strings.

Additionally to the above, improve documentation for the call method.

* Improve consistency in documentation
* Improve rubocop assessment in new code we are responsible for
        - Improve documentation

* Correct scheduled post parse error semantics
        - Return 422, not 400
        - Respond with a useful error message

* Allow creating limited visibility posts from the API again

* Add appropriate yard tags to documentation

* Correct erronous documentation, copy-editing it somewhat
	- Use a labelled list for Status#visibility
	- Improve specificity of "make a note of that"

* Remove PostStatusService#visibility_invalid?
	- !visibility_invalid? -> !visibility_valid?