RFR: JDK-8317612: ChoiceFormat and MessageFormat constructors call non-final public method

[Justin Lu]

Please review this PR and [CSR](https://bugs.openjdk.org/browse/JDK-8317630) which makes the implications of overriding _ChoiceFormat::applyPattern_ and _MessageFormat::applyPattern_ apparent by adding a implSpec tag to the method. 

That is, the constructors of both ChoiceFormat and MessageFormat call the public and non-final applyPattern method; any changes in the applyPattern of a subclass will be reflected in the constructors as well.

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Commit messages:
 - code tag for MessageFormat::applyPattenr as well
 - Review: use 'may' wording
 - init

Changes: https://git.openjdk.org/jdk/pull/16064/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317612
  Stats: 7 lines in 2 files changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16064.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16064/head:pull/16064

PR: https://git.openjdk.org/jdk/pull/16064