jquery-core/selecting-elements: replace info by links to api docs

This replaces the parts about the `:<type>` pseudo selectors by a list & links to api.jquery.com. It was too repetitive and kinda duplicative of what's up on the api docs.

Would fix https://github.com/jquery/learn.jquery.com/issues/338
Refs https://github.com/jquery/learn.jquery.com/issues/180