RFR: 8364317: Explicitly document some assumptions of StringUTF16

[Raffaello Giulietti]

On Tue, 29 Jul 2025 21:09:59 GMT, Chen Liang <liach at openjdk.org> wrote:

> In #24773, people were concerned that the layout of a UTF16 byte array and a char array may be incompatible. In fact, they are - they are asserted in a corner in `LibraryCallKit::inline_string_char_access` in `library_call.cpp`.
> 
> In addition, another frequent error I see is that contributors have confused the meaning of indices in StringUTF16 - the indices are always in char array indices. I think we should make these explicit to help future maintenance.

A good initiative, thanks @liach.
Maybe some words about endianness would be useful as well.

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PR Review: https://git.openjdk.org/jdk/pull/26541#pullrequestreview-3071115533