clean-temp: Clarify what it does. * lib/clean-temp.h: Add more comments. * doc/posix-functions/mkstemp.texi: Tweak reference to 'clean-temp' module. * doc/glibc-functions/mkostemp.texi: Mention 'clean-temp' here too. * doc/glibc-functions/mkstemps.texi: Likewise. * doc/glibc-functions/mkostemps.texi: Likewise.