RFR: 8345292: Improve javadocs for MemorySegment::getStrings defining word boundary cases [v5]

[Per Minborg]

On Mon, 16 Jun 2025 16:58:09 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> 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 incrementally with two additional commits since the last revision:
> 
>  - Remove imp note
>  - Add text on N octets

I've proposed a new top-level section:

<img width="1128" height="412" alt="image" src="https://github.com/user-attachments/assets/c498554b-fb67-4937-92b3-0cf18dfeed16" />

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PR Comment: https://git.openjdk.org/jdk/pull/25715#issuecomment-3154819798