docs: update the unmount propagation rule

Section 5f still describes the rule propagate_umount() used before
commit f0d0ba19985d ("Rewrite of propagate_umount()"). It states that a
mount that receives the unmount by propagation is left alone as soon as
it has any submount. That's not true anymore.

A propagated unmount unmounts a mount with sub-mounts as long as every
sub-mount gets unmounted together with it. This is the case when the
sub-mounts are unmounted by the same propagation. Only a sub-mount that
cannot be unmounted keeps its parent mounted.

The section also only describes a single mount without sub-mounts. But
lazy unmounts take a tree and every mount of the tree propagates its
unmount from its parent mount.

Link: https://patch.msgid.link/20260923-work-mount-fixes-v1-8-f424cf8d3242@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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