t/lib-submodule-update: move a test to the right section

t/lib-submodule-update: move a test to the right section

The test "$command: submodule branch is not changed, detach HEAD
instead" is in the "Appearing submodule" section of
test_submodule_recursing_with_args_common(), but this test updates a
submodule; it does not test a transition from a state with no submodule
to a state with a submodule.

As such, for consistency, move it to the "Modified submodule" section of
the same function. While at it, add a comment describing the test.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>