Update polyfill-next-selector docs for #637

Reviewer: @robdodson 

This change attempts to revise the `polyfill-next-selector` docs to only recommend using the feature when doing something that would not work if `::content` were removed. As part of this, I've tried to trim down some of the previous examples that focused on the `::content` usecase. If I misunderstood what we were aiming for here, feel free to holler.

https://github.com/Polymer/docs/issues/637