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 f35a6d3bce (Teach core object handling functions
about gitlinks, 2007-04-09). E.g. the discussion of --long added in
f35a6d3bce (Teach core object handling functions about gitlinks,
2007-04-09) didn't explicitly mention them.

But this documentation added in 455923e0a1 (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>

Documentation/git-ls-tree.txt

@@ -151,11 +151,11 @@ names can be used:
 objectmode::
 	The mode of the object.
 objecttype::
-	The type of the object (`blob` or `tree`).
+	The type of the object (`commit`, `blob` or `tree`).
 objectname::
 	The name of the object.
 objectsize[:padded]::
-	The size of the object ("-" if it's a tree).
+	The size of a `blob` object ("-" if it's a `commit` or `tree`).
 	It also supports a padded format of size with "%(objectsize:padded)".
 path::
 	The pathname of the object.