RFR: JDK-8313204: Inconsistent order of sections in generated class documentation

[Jonathan Gibbons]

Please review a minor change to `ClassWriter` to make the order of sections consistent across member-kinds and between summary and detail sections. The change to `ClassWriter` is just to reorder the sections, and to add some explanatory comments.

One test detected the change and is therefore updated.
Another test is updated to include a check for the section ordering.

Note: the changes to `ClassWriter` may soon be superseded by work on [JDK-8312201](https://bugs.openjdk.org/browse/JDK-8312201). The primary goal of this PR is to establish the preferred order and update the tests accordingly.

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Commit messages:
 - JDK-8313204: Inconsistent order of sections in generated class documentation

Changes: https://git.openjdk.org/jdk/pull/15046/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15046&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313204
  Stats: 67 lines in 3 files changed: 48 ins; 13 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/15046.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15046/head:pull/15046

PR: https://git.openjdk.org/jdk/pull/15046