RFR: JDK-8275273: Add missing HtmlStyle documentation

[Hannes Wallnöfer]

Please review this simple change to add doc comments to the so far undocumented `HtmlStyle` constants. I also removed one more unused constant (memberNameLabel).

-------------

Commit messages:
 - JDK-8275273: Add missing HtmlStyle documentation

Changes: https://git.openjdk.java.net/jdk/pull/5944/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5944&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275273
  Stats: 165 lines in 2 files changed: 135 ins; 28 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5944.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5944/head:pull/5944

PR: https://git.openjdk.java.net/jdk/pull/5944