RFR: 8306036: Use @apiNote in String.toLowerCase, String.toUpperCase [v2]

[Eirik Bjorsnos]

> Please review this PR which suggests to use `@apiNote` in the `String.toLowerCase()` and `String.toUpperCase()` API docs.
> 
> These methods include important usage notes which today are encoded using `<b>Note:</b>`.

Eirik Bjorsnos has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into to-ul-case-apinote
 - Use @apiNote in String.toLowerCase, String.toUpperCase

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13487/files
  - new: https://git.openjdk.org/jdk/pull/13487/files/a9ba49e5..044ad67d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13487&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13487&range=00-01

  Stats: 1259 lines in 134 files changed: 69 ins; 130 del; 1060 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