RFR: 8364317: Explicitly document some assumptions of StringUTF16 [v2]

[Chen Liang]

> 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.

Chen Liang has updated the pull request incrementally with one additional commit since the last revision:

  Add paragraph for endianness and layout

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26541/files
  - new: https://git.openjdk.org/jdk/pull/26541/files/933557ef..1b052e9e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26541&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26541&range=00-01

  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26541.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26541/head:pull/26541

PR: https://git.openjdk.org/jdk/pull/26541