RFR: JDK-8258602: JavaDoc field summary does not indicate final modifier

[Hannes Wallnöfer]

I found that adding the `final` modifier does not add too much visual noise to the documentation. I didn't find any occurrence where there were more than two modifiers on a member summary (usually either `static final` or `protected final`). 

On the other hand, I do think the `final` modifier adds important information about the member. Although this is more true for fields than for other members, for reasons of consistency I decided to show it for all kinds of members except for those where it is defined implicitly (such as enums).

To my surprise I also noticed that there weren't any test cases for field summaries, so I added a new test.

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Commit messages:
 - JDK-8258602: JavaDoc field summary does not indicate final modifier

Changes: https://git.openjdk.java.net/jdk/pull/3716/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3716&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258602
  Stats: 174 lines in 8 files changed: 159 ins; 0 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3716.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3716/head:pull/3716

PR: https://git.openjdk.java.net/jdk/pull/3716