RFR: 8336043: Add quality of implementation discussion to Object.{equals, toString, hashCode}

[Joe Darcy]

First pass at adding some quality of implementation discussions around the overridable methods of Object.

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Commit messages:
 - Appease jcheck.
 - JDK-8336043: Add quality of implementation discussion to Object.{equals, toString, hashCode}

Changes: https://git.openjdk.org/jdk/pull/20128/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20128&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336043
  Stats: 52 lines in 1 file changed: 52 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20128.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20128/head:pull/20128

PR: https://git.openjdk.org/jdk/pull/20128