proc.5: Document change of /proc/PID/ns/* files to symlinks in Linux 3.8

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

man5/proc.5

@@ -877,6 +877,25 @@ the namespace will remain alive,
 even if all processes in the namespace terminate.
 The file descriptor can be passed to
 .BR setns (2).
+
+In Linux 3.7 and earlier, these files were visible as hard links.
+Since Linux 3.8, they appear as symbolic links.
+If two processes are in the same namespace, then the inode numbers of their
+.IR /proc/[pid]/ns/xxx
+symbolic links will be the same; an application can check this using the
+.I stat.st_ino
+field returned by
+.BR stat (2).
+The content of this symbolic link is a string containing
+the namespace type and inode number as in the following example:
+
+.in +4n
+.nf
+$ \fBreadlink /proc/$$/ns/uts\fP
+uts:[4026531838]
+.fi
+.in
+
 .TP
 .IR /proc/[pid]/ns/ipc " (since Linux 3.0)"
 This file is a handle for the IPC namespace of the process;