proc.5: Document /proc/PID/status 'Umask' field

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/proc.5

@@ -1957,6 +1957,7 @@ Here's an example:
 
 .RB "$" " cat /proc/$$/status"
 Name:   bash
+Umask:  0022
 State:  S (sleeping)
 Tgid:   17248
 Ngid:   0
@@ -2012,6 +2013,10 @@ The fields are as follows:
 .IR Name :
 Command run by this process.
 .IP *
+.IR Umask :
+Process umask, expressed in octal with a leading zero.
+(Since Linux 4.7.)
+.IP *
 .IR State :
 Current state of the process.
 One of