[jdk18] RFR: JDK-8273452: DocTrees.getDocCommentTree should be specified as idempotent [v2]

[Hannes Wallnöfer]

> Please review a doc-only change to add implementation notes to the `DocTrees.getDocCommentTree` methods, some of which return the same `DocCommentTree` instance on repeated invocation and some don't. I decided to use `@implNote` instead of `@implSpec` as usually I wouldn't expect object identity to be part of a Java API specification.
> 
> I verified the described behavior using code analysis and enhancing existing tests. The latter are not included in the commit as this is a `noreg-doc` issue.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8273452: review feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk18/pull/36/files
  - new: https://git.openjdk.java.net/jdk18/pull/36/files/f69a1052..66cbd42b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk18&pr=36&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk18&pr=36&range=00-01

  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk18/pull/36.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18 pull/36/head:pull/36

PR: https://git.openjdk.java.net/jdk18/pull/36