RFR: docs JDK-8220261: fix headings in java.base

[Jonathan Gibbons]

Please review a medium-sized but conceptually simple patch to fix most 
of the headings in the generated documentation for java.base, as 
described in [1]. This does not address the headings in 
java.util.concurrent which are reported separately, in JDK-8220248.

The intent is to ensure that the headings in the generated pages are 
well-structured, with no gaps, and correct implicit nesting. Generally, 
the changes fall into three kinds:

  * Headings in doc-comments for modules, packages and types start at <h2>
  * Headings in doc comments for constructors, fields, methods and other
    members start at <h4>
  * Headings in package.html and doc-files/*.html start at <h1>

The headings have been verified with the doccheck utility.

We cannot re-enable doclint accessibility checking for java.base until 
JDK-8220248 has also been fixed.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8220261
Webrev: http://cr.openjdk.java.net/~jjg/8220261/webrev.00/index.html

[1]: https://mail.openjdk.java.net/pipermail/jdk-dev/2019-March/002671.html