ls-tree doc: document interaction with submodules

ls-tree doc: document interaction with submodules

The ls-tree documentation had never been updated after it learned to
interact with submodules to explicitly mention them. The initial
support was added in f35a6d3bce7 (Teach core object handling functions
about gitlinks, 2007-04-09). E.g. the discussion of --long added in
f35a6d3bce7 (Teach core object handling functions about gitlinks,
2007-04-09) didn't explicitly mention them.

But this documentation added in 455923e0a15 (ls-tree: introduce
"--format" option, 2022-03-23) had no such excuse, and was actively
misleading by providing an exhaustive but incomplete list of object
types we'd emit.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>