RFR: JDK-8280738: Minor cleanup for HtmlStyle

[Jonathan Gibbons]

Please review a simple update to reorder some members of the enum `HtmlStyle`, and to make some minor updates to the comments.

This is to facilitate automated extraction of information into an upcoming new spec for the output from the standard doclet.

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Commit messages:
 - JDK-8280738: Minor cleanup for HtmlStyle

Changes: https://git.openjdk.java.net/jdk/pull/7235/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7235&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280738
  Stats: 82 lines in 1 file changed: 54 ins; 26 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7235.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7235/head:pull/7235

PR: https://git.openjdk.java.net/jdk/pull/7235