RFR: 8306036: Use @apiNote in String.toLowerCase, String.toUpperCase

[Eirik Bjorsnos]

Please review this PR which suggests to use `@apiNote` in the `String.toLowerCase()` and `String.toUpperCase()` API docs.

These methods include import usage notes which today are encoded using `<b>Note:</b>`.

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Commit messages:
 - Use @apiNote in String.toLowerCase, String.toUpperCase

Changes: https://git.openjdk.org/jdk/pull/13487/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13487&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306036
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13487.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13487/head:pull/13487

PR: https://git.openjdk.org/jdk/pull/13487