RFR: 8220002: Improve anchor definitions in generated files

[Hannes Wallnöfer]

Please review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8220002
Webrev: http://cr.openjdk.java.net/~hannesw/8220002/webrev.01/
API docs: http://cr.openjdk.java.net/~hannesw/8220002/api.01/

In places where a <section> tag is available I always moved the id attribute there instead of the heading. This required a bit more changes in some places, but was necessary to get the anchoring position right (see for example the section and individual member anchors in type pages, both of which are now just right IMO).

This patch should cover all occurrences of <a id=…> tags in generated docs, but there are still dozens in static HTML files. I’ll file a separate issue for those.

Hannes