RFR: 8340983: Use index and definition tags in Object and Double

[Joe Darcy]

Simple change to add a few more index tags on terms related to equality and equivalence.

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Commit messages:
 - JDK-8340983: Use index and definition tags in Object and Double

Changes: https://git.openjdk.org/jdk/pull/21195/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21195&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340983
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21195.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21195/head:pull/21195

PR: https://git.openjdk.org/jdk/pull/21195