doc: update buffer doc with swap16() note to indicate big endian
This simple doc update PR attempts to resolve #12813 (closed). If the PR is incorrect, happy to update under advice, or I can close the PR if it's irrelevant. Thanks.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines