doc: Mention reason for replacement on glibc/Linux systems.

doc: Mention reason for replacement on glibc/Linux systems.

* doc/posix-functions/dprintf.texi: Mention the problem with special
'long double' values.
* doc/posix-functions/fprintf.texi: Likewise.
* doc/posix-functions/printf.texi: Likewise.
* doc/posix-functions/snprintf.texi: Likewise.
* doc/posix-functions/sprintf.texi: Likewise.
* doc/posix-functions/vdprintf.texi: Likewise.
* doc/posix-functions/vfprintf.texi: Likewise.
* doc/posix-functions/vprintf.texi: Likewise.
* doc/posix-functions/vsnprintf.texi: Likewise.
* doc/posix-functions/vsprintf.texi: Likewise.
* doc/posix-functions/fcntl.texi: Add glibc/Linux to the list of
platforms with F_DUPFD_CLOEXEC problems.
* doc/posix-functions/glob.texi: Mention which platforms are affected
by the problem with symbolic links.
* doc/posix-functions/linkat.texi: Mention the problem with
AT_SYMLINK_FOLLOW on Linux.