<i18n dev> RFR: 8336787: Examine java.text.Format API for implSpec usage

[Justin Lu]

As discussed in https://bugs.openjdk.org/browse/JDK-8335834, there are occurrences in the java.text.Format API that would benefit from the implSpec (and apiNote) tag. 

This is a doc-only change, and a CSR has also been filed.

I did not think "_Subclasses that support fields should override this and create an ..._" sounded right under the `implSpec` tag, and moved it to an `apiNote` tag.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/20303/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20303&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336787
  Stats: 11 lines in 1 file changed: 5 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20303.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20303/head:pull/20303

PR: https://git.openjdk.org/jdk/pull/20303