RFR: 8261123: Augment discussion of equivalence classes in Object.equals and comparison methods [v6]

[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:

  Fix typo.

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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/41102f52..03f01e65

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2471&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2471&range=04-05

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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