RFR: JDK-8267575: Add new documentation group in HtmlStyle

[Jonathan Gibbons]

Please review a relatively simple primarily-doc update for HtmlStyle.

The change is mostly grouping and commenting items.

Some existing comments were incorrect and have been updated.

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Commit messages:
 - JDK-8267575: Add new documentation group in HtmlStyle

Changes: https://git.openjdk.java.net/jdk/pull/4155/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4155&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267575
  Stats: 89 lines in 1 file changed: 75 ins; 12 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4155.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4155/head:pull/4155

PR: https://git.openjdk.java.net/jdk/pull/4155