RFR: JDK-8220379: Fix doclint handling of headings

[Jonathan Gibbons]

Please review a moderately simple fix to improve doclint's checking of 
headings in doc comments.

Build folk: by default, the improved checking causes build issues until 
the doc comments in some modules can be updated. To workaround that, the 
doclint options in the build have been modified. It is assumed we will 
re-enable the checks as the issues in each affected module get addressed.

javadoc/doclint folk: the new guidelines of using h2 or h4 as the base 
heading as appropriate are now hard-coded into doclint. In addition, the 
error messages have been improved, and a new check added, for using a 
heading that is too low for the context. Internally, some names have 
been improved to reflect the current terminology.  Tests have been 
updated as appropriate.

-- Jon