doc: add note about mkdtemp() platform differences
Fixes: https://github.com/nodejs/node/issues/26435
I'm not sure how strongly anyone feels about this, but I could potentially emit a warning if the provided prefix string ends with X.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines