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 <[email protected]>
Signed-off-by: Chen Linxuan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jonathan Corbet <[email protected]>

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