During recent work to fix headings in generated docs, it became clear that the previous guidelines for using headings in documentation comments were flawed, *and* that doclint imperfectly checked those guidelines.

The new/revised guidelines are:

* Headings in documentation comments for all modules, packages and types (including nested types) shuld begin at <h2>.
* Headings in members should begin at <h4>.

Previously, and incorrectly, doclint assumed that all headings should begin at <h3>, and while it checked for messing headings, it did not check for low headings.