RFR: 5066247: Refine the spec of equals() and hashCode() for j.text.Format classes [v2]

[Justin Lu]

> Please review this PR which refines the spec of `equals()` and `hashCode()` in `java.text.Format` related classes.
> 
> The current spec for most of these methods is either  "_Overrides <method_name>_" or are incomplete/wrong (i.e. see `ChoiceFormat`).
> 
> This fix adjusts the spec to provide a consistent definition for the overridden methods and specify what is being compared/used to generate a hash code value.
> 
> For implementations that use at most a few fields, the values are stated, otherwise a more general term is used as a substitution (i.e. see `DecimalFormat`).

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  Reflect review comments: Make implementers aware of default classIdentity notion, generalize all hashcodes/equals, adjust hashCode spec to be more accurate

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15459/files
  - new: https://git.openjdk.org/jdk/pull/15459/files/84bd4708..3e2d6745

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15459&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15459&range=00-01

  Stats: 58 lines in 9 files changed: 29 ins; 4 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/15459.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15459/head:pull/15459

PR: https://git.openjdk.org/jdk/pull/15459