RFR: 8336679: Add @implSpec for the default implementations in Process.waitFor()

[Naoto Sato]

This is a simple doc-only change that follows up [JDK-8336479](https://bugs.openjdk.org/browse/JDK-8336479). A corresponding CSR has also been drafted.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/20285/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336679
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20285/head:pull/20285

PR: https://git.openjdk.org/jdk/pull/20285