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

[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.

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Commit messages:
 - Merge branch 'master' into 8261123
 - Initial changes for JDK-8261123.

Changes: https://git.openjdk.java.net/jdk/pull/2471/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2471&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261123
  Stats: 80 lines in 4 files changed: 38 ins; 14 del; 28 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