RFR: 8174840: Elements.overrides does not check the return type of the methods

[Nizar Benalla]

Please review this PR to clarify the documentation of `Elements.overrides` through an `@apiNote`, this PR is essentially a doc-only change. This PR supersedes https://github.com/openjdk/jdk/pull/22244.

TIA.

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Commit messages:
 - Respond to feedback around the wording.
 - Respond to feedback. Using Jan's comment to not imply particular requirement on the implementation.
 - (C) 2025
 - Initial commit obtained from a cherry pick/squash of a now superseded PR.

Changes: https://git.openjdk.org/jdk/pull/22920/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8174840
  Stats: 126 lines in 3 files changed: 125 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22920.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22920/head:pull/22920

PR: https://git.openjdk.org/jdk/pull/22920