RFR: JDK-8323760 putIfAbsent documentation conflicts with itself

[John Hendrikx]

Update the documentation for `@return` tag of `putIfAbsent` to match the main description. `putIfAbsent` uses the same wording as `put` for its `@return` tag, but that is incorrect.  `putIfAbsent` never returns the **previous** value, as the whole point of the method is not the replace the value if it was present.  As such, if it returns a value, it is the **current** value, and in all other cases it will return `null`.

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Commit messages:
 - Improve putIfAbsent documentation

Changes: https://git.openjdk.org/jdk/pull/17438/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17438&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323760
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17438.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17438/head:pull/17438

PR: https://git.openjdk.org/jdk/pull/17438