[jdk16] RFR: JDK-8251200: False positive messages about missing comments for serialization

[Jonathan Gibbons]

Using the `@serial` tag, a class can explicitly opt-in to being documented on the serialized form page.  However, a class that does not want to opt-in should not be required to have a doc comment, and should not trigger the "missing comment" warning if there is no comment.

The fix is to check for the presence of the comment on a class before analyzing it to see if it contains `@serial` tags.

The code in `Utils.getDocCommentTree0` is slightly refactored to allow a new predicate `hasDocCommentTree` to be added.

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Commit messages:
 - JDK-8251200: False positive messages about missing comments for serialization

Changes: https://git.openjdk.java.net/jdk16/pull/70/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=70&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251200
  Stats: 138 lines in 3 files changed: 115 ins; 21 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/70.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/70/head:pull/70

PR: https://git.openjdk.java.net/jdk16/pull/70