doc: improve randomness discussion Inspired by comments from Jeffrey Walton in: https://lists.gnu.org/r/bug-gnulib/2020-06/msg00002.html * doc/glibc-functions/getentropy.texi (getentropy): * doc/glibc-functions/getrandom.texi (getrandom): Improve discussion of problems with "random" data, and cite Ristenpart & Yilek. Also, mention GRND_INSECURE.