doc: Update for glibc 2.36.
* doc/posix-functions/c8rtomb.texi: Update.
* doc/posix-functions/mbrtoc8.texi: Update.
* doc/glibc-functions/arc4random.texi: New file.
* doc/glibc-functions/arc4random_buf.texi: New file.
* doc/glibc-functions/arc4random_uniform.texi: New file.
* doc/glibc-functions/fsconfig.texi: New file.
* doc/glibc-functions/fsmount.texi: New file.
* doc/glibc-functions/fsopen.texi: New file.
* doc/glibc-functions/fspick.texi: New file.
* doc/glibc-functions/mount_setattr.texi: New file.
* doc/glibc-functions/move_mount.texi: New file.
* doc/glibc-functions/open_tree.texi: New file.
* doc/glibc-functions/pidfd_getfd.texi: New file.
* doc/glibc-functions/pidfd_open.texi: New file.
* doc/glibc-functions/pidfd_send_signal.texi: New file.
* doc/glibc-functions/process_madvise.texi: New file.
* doc/glibc-functions/process_mrelease.texi: New file.
* doc/gnulib.texi: Include them.
(Glibc sys/pidfd.h): New section.