getrandom, getentropy: Mention the crypto/gc-random module. Suggested by Simon Josefsson in <https://lists.gnu.org/archive/html/bug-gnulib/2020-05/msg00400.html>. * doc/glibc-functions/getrandom.texi: Mention the quality issues and the crypto/gc-random module. * doc/glibc-functions/getentropy.texi: Likewise.