proc.5: Note capability requirements for writing to /proc/PID/{uid_map,gid_map}

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

man5/proc.5

@@ -1947,6 +1947,15 @@ in any other lines.
 .IP
 Writes that violate the above rules fail with the error
 .BR EINVAL .
+
+In order to write to the
+.I /proc/[pid]/uid_map
+.RI ( /proc/[pid]/gid_map )
+file, a process must have the
+.BR CAP_SETUID
+.RB ( CAP_SETGID )
+capability in the user namespace of the process
+.IR pid .
 .TP
 .IR /proc/[pid]/wchan " (since Linux 2.6.0)"
 The symbolic name corresponding to the location