RFR: 8345292: Improve javadocs for MemorySegment::getStrings defining word boundary cases [v6]

[Per Minborg]

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

Per Minborg 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 eight additional commits since the last revision:

 - Add a top-level section on string conversion
 - Merge branch 'master' into ms-getstrings-boundary-doc
 - Remove imp note
 - Add text on N octets
 - Improve wording
 - Update src/java.base/share/classes/java/lang/foreign/MemorySegment.java
   
   Co-authored-by: Jorn Vernee <JornVernee at users.noreply.github.com>
 - Update after comments
 - Add info for multi-octet Charsets

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/25715/files
  - new: https://git.openjdk.org/jdk/pull/25715/files/5f5b1473..72ee9bb7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25715&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25715&range=04-05

  Stats: 98262 lines in 2417 files changed: 56529 ins; 27369 del; 14364 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