RFR: 8306039: ParameterizedType.getOwnerType() documentation is incomplete about null result

[Chen Liang]

Clarify that `ParameterizedType.getOwnerType()` always return `null` in a few scenarios.

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Commit messages:
 - 8306039: ParameterizedType.getOwnerType() documentation is incomplete about null result

Changes: https://git.openjdk.org/jdk/pull/19977/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19977&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306039
  Stats: 22 lines in 3 files changed: 2 ins; 8 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/19977.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19977/head:pull/19977

PR: https://git.openjdk.org/jdk/pull/19977