Document migration path for obsolescent functions.
* doc/glibc-functions/timespec_get.texi: Add reference to ISO C.
* doc/posix-functions/_tolower.texi: Recommend use of tolower.
* doc/posix-functions/_toupper.texi: Recommend use of toupper.
* doc/posix-functions/asctime.texi: Recommend use of strftime.
* doc/posix-functions/asctime_r.texi: Likewise.
* doc/posix-functions/ctime.texi: Recommend use of localtime_r and
strftime.
* doc/posix-functions/ctime_r.texi: Likewise.
* doc/posix-functions/ftw.texi: Recommend use of fts.
* doc/posix-functions/getitimer.texi: Recommend use of timer_gettime.
* doc/posix-functions/gets.texi: Recommend use of fgets.
* doc/posix-functions/gettimeofday.texi: Recommend use of gettime or
timespec_get.
* doc/posix-functions/inet_addr.texi: Recommend use of inet_pton.
* doc/posix-functions/inet_ntoa.texi: Recommend use of inet_ntop.
* doc/posix-functions/pthread_getconcurrency.texi: Recommend no-op.
* doc/posix-functions/pthread_setconcurrency.texi: Recommend no-op.
* doc/posix-functions/rand_r.texi: Recommend use of random_r.
* doc/posix-functions/setitimer.texi: Recommend use of timer_create and
timer_settime.
* doc/posix-functions/setpgrp.texi: Recommend use of setpgid or setsid.
* doc/posix-functions/sighold.texi: Recommend use of sigprocmask.
* doc/posix-functions/sigignore.texi: Recommend use of sigaction.
* doc/posix-functions/siginterrupt.texi: Recommend use of sigaction.
* doc/posix-functions/sigpause.texi: Recommend use of sigsuspend.
* doc/posix-functions/sigrelse.texi: Recommend use of sigprocmask.
* doc/posix-functions/sigset.texi: Recommend use of sigaction.
* doc/posix-functions/tempnam.texi: Recommend use of mkstemp.
* doc/posix-functions/ulimit.texi: Recommend use of getrlimit and
setrlimit.
* doc/posix-functions/utime.texi: Recommend use of utimens.