[PATCH v3] proc.5: document 'subset' mount option.

[technoboy85@gmail.com]

From: Matteo Croce <teknoraver@meta.com>

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
	#

Signed-off-by: Matteo Croce <teknoraver@meta.com>
---
 man/man5/proc.5 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/man/man5/proc.5 b/man/man5/proc.5
index 8022ca49e..cdb4aa859 100644
--- a/man/man5/proc.5
+++ b/man/man5/proc.5
@@ -105,7 +105,12 @@ This group should be used instead of approaches such as putting
 nonroot users into the
 .BR sudoers (5)
 file.
-.\"
+.RE
+.TP
+.BR subset = pid " (since Linux 5.8)"
+.\" commit 6814ef2d992af09451bbeda4770daa204461329e
+Show only the specified subset of procfs hiding all top level files
+and directories in the procfs that are not related to tasks.
 .SS Overview
 Underneath
 .IR /proc ,
-- 
2.43.0