Orthogonal to this PR, but my Sphinx was throwing up a warning from there not being a linebreak here.
A bit of other miscellaneous cleanup not related to astropy-helpers
Discussion of astropy-helpers in many places where appropriate. Especially in the docs guide since all the sphinx extensions now live in astropy-helpers.  Also links to APE 4 in several places where it describes astropy-helpers usage
minor typo fixes/updates for astropy-helper doc changes
Merge pull request #2545 from embray/docs/astropy-helpers

Includes an additional commit by @eteq added manually