Cleanup Guides Guidelines [ci skip]

Most of these changes are copy edits to make this Guide easier to read. Feedback is welcome.

* Copy editing for clarity in several locations
* Rename "Titles" section to "Headings"
* Fix incorrect anchor in link to API Documentation Guidelines for filenames
@zzak The way I read it, "heading" is the more generic term for a guide's title, the name of a section, or the name of a sub section. I would call the highest-level heading of the guide its "title".