RFR: 8345292: Improve javadocs for MemorySegment::getStrings defining word boundary cases

[Per Minborg]

This PR proposes to improve the 'MemorySegment.getString(long offset, Charset charset)` method documentation with respect to multi-octet concerns.

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

Commit messages:
 - Add info for multi-octet Charsets

Changes: https://git.openjdk.org/jdk/pull/25715/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345292
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25715.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25715/head:pull/25715

PR: https://git.openjdk.org/jdk/pull/25715