Documentation: update the description of TracerPid in procfs.rst

When the tracer of process is outside of current pid namespace, field
`TracerPid` in /proc/<pid>/status will be 0, too, just like this process
not have been traced.

This is because that function `task_pid_nr_ns` used to get the pid of
tracer will return 0 in this situation.
Co-authored-by: Yuan Haisheng <heysion@deepin.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Link: https://lore.kernel.org/r/20221102081517.19770-1-chenlinxuan@uniontech.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/proc.rst

@@ -245,7 +245,8 @@ It's slow but very precise.
  Ngid                        NUMA group ID (0 if none)
  Pid                         process id
  PPid                        process id of the parent process
- TracerPid                   PID of process tracing this process (0 if not)
+ TracerPid                   PID of process tracing this process (0 if not, or
+                             the tracer is outside of the current pid namespace)
  Uid                         Real, effective, saved set, and  file system UIDs
  Gid                         Real, effective, saved set, and  file system GIDs
  FDSize                      number of file descriptor slots currently allocated