RFR: 8338140: (str) Add notes to String.trim and String.isEmpty pointing to newer APIs

[Stuart Marks]

Add a note to String.trim pointing to the String.strip family of methods.

Add notes cross-linking String.isBlank and String.isEmpty.

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

Commit messages:
 - Updates to String javadoc.

Changes: https://git.openjdk.org/jdk/pull/25762/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25762&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338140
  Stats: 16 lines in 1 file changed: 9 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/25762.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25762/head:pull/25762

PR: https://git.openjdk.org/jdk/pull/25762