Add docs for paginate_links $args argument

[atimmer]

paginate_links has a lot of documentation in words as to what values are possible to send in the $args array but it's all natural text, there is no overview of arguments for a quick reference.

Compare ​https://developer.wordpress.org/reference/functions/paginate_links/
With ​http://codex.wordpress.org/Function_Reference/paginate_links

[atimmer]

31433.diff adds this documentation. This is based on the codex. It probably needs some feedback.

[DrewAPicture]

In 31854:

Document default arguments for paginate_links() as a hash notation.

Props atimmer.
Fixes #31433.