proc.5: Rework the description of /proc/PID/mountinfo parent-ID field

proc.5: Rework the description of /proc/PID/mountinfo parent-ID field

After comments from Miklos, and further digging in the kernel
source that showed that chroot() can also result in "hidden"
parent-IDs in mountinfo, I've revised the description of
mountinfo.

In fs/proc_namespace.cs::how_mountinfo() there is:

        /* mountpoints outside of chroot jail will give SEQ_SKIP on this */
        err = seq_path_root(m, &mnt_path, &p->root, " \t\n\\");
        if (err)
                goto out;

I instrumented the 'if (err)' code path with printk()
to show that there is indeed a record corresponding to the
parent-ID for the process root that is being skipped.

Reported-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>