proc.5: Document 'subset' mount option.

proc.5: Document 'subset' mount option.

The 'subset=pid' option was added in commit 6814ef2d, document it.
This option mounts a procfs where only the numeric directories related
to the PIDs are present.

	# mount -t proc proc_pid pid -o subset=pid
	# mount |grep -w proc_pid
	proc_pid on /tmp/proc/pid type proc (rw,relatime,subset=pid)

	# ll -d pid/{1,$$,cmdline,version}
	ls: cannot access 'pid/cmdline': No such file or directory
	ls: cannot access 'pid/version': No such file or directory
	dr-xr-xr-x. 9 root root 0 May 14 09:43 pid/1
	dr-xr-xr-x. 9 root root 0 May 14 09:43 pid/25146

The only non-numeric entries in that procfs instance are
'self' and 'thread-self':

	# ls pid |grep -vx '[[:digit:]]*'
	self
	thread-self
	#

Cc: "G. Branden Robinson" <branden@debian.org>
Signed-off-by: Matteo Croce <teknoraver@meta.com>
Message-ID: <20240514202819.95347-1-technoboy85@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>