State explicitly that holding a /proc/pid file descriptor open does
not reserve the PID. Also note that in the event of PID reuse, these
open file descriptors refer to the old, now-dead process, and not the
new one that happens to be named the same numeric PID.
Signed-off-by: Daniel Colascione <dancol@google.com> Acked-by: Michal Hocko <mhocko@suse.com> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>