blockdev: Fix active commit choice

We have to perform an active commit whenever the top node has a parent
that has taken the WRITE permission on it.

This means that block-commit's @backing-file parameter is no longer
allowed for such nodes, and that users will have to issue a
block-job-complete command.  Neither should pose a problem in practice,
because this case was basically just broken until now.

(Since this commit already touches block-commit's documentation, it also
moves up the chunk explaining general block-commit behavior that for
some reason was situated under @backing-file.)

Signed-off-by: Max Reitz <mreitz@redhat.com>

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