RFR: JDK-8294539: Augment discussion of equivlance relations on floating-point values

[Joe Darcy]

While the floating-point == operation is *not* an equivalence relation, there are useful equivalence relations that can be defined over floating-point values. Text is added to java.lang.Double to discuss and name those relations.

-------------

Commit messages:
 - JDK-8294539: Augment discussion of equivlance relations on floating-point values

Changes: https://git.openjdk.org/jdk/pull/10498/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10498&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294539
  Stats: 38 lines in 1 file changed: 37 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10498.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10498/head:pull/10498

PR: https://git.openjdk.org/jdk/pull/10498