docs: explain why path.posix.normalize does not replace windows slashes
Add section to path docs that explains that path.posix.normalize does not replace Windows slashes with POSIX slashes because POSIX does not recognize \ as a valid path separator.
Fixes: https://github.com/nodejs/node/issues/12298
Added a note about directory separator conversion under path.posix.
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc