RFR: 8261123: Augment discussion of equivalence classes in Object.equals and comparison methods [v5]

[Joe Darcy]

> A follow-up of sorts to JDK-8257086, this change aims to improve the discussion of the relationship between Object.equals and compareTo and compare methods. The not-consistent-with-equals natural ordering of BigDecimal get more explication too. While updating Object, I added some uses of apiNote and implSpec, as appropriate. While a signum method did not exist when Comparable was added, it has existed for many years so I removed obsolete text that defined a "sgn" function to compute signum.
> 
> Once the changes are agreed to, I'll reflow paragraphs and update the copyright years.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to more review feedback.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2471/files
  - new: https://git.openjdk.java.net/jdk/pull/2471/files/a7f1b28b..41102f52

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2471&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2471&range=03-04

  Stats: 12 lines in 3 files changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2471.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2471/head:pull/2471

PR: https://git.openjdk.java.net/jdk/pull/2471