Describe Submodule.__init__ parent_commit parameter

This includes a brief description of the Submodule.__init__
parent_commit parameter in its docstring, rather than only
referring to the set_parent_commit method, whose semantics differ
due to conversation and validation, and which accepts more types
than just Commit or None.

The wording is based on wording in set_parent_commit, adjusted for
the difference in types, and set_parent_commit remains reference
for further details.

This builds on 1f03e7f (gitpython-developers#1859) in improving the situation described
in gitpython-developers#1869.

git/objects/submodule/base.py

@@ -143,7 +143,9 @@ def __init__(
             See the `url` parameter.
 
         :param parent_commit:
-            See :meth:`set_parent_commit`.
+            The :class:`~git.objects.commit.Commit` whose tree is supposed to contain
+            the ``.gitmodules`` blob, or ``None`` to always point to the most recent
+            commit. See :meth:`set_parent_commit` for details.
 
         :param url:
             The URL to the remote repository which is the submodule.
@@ -1260,7 +1262,7 @@ def set_parent_commit(self, commit: Union[Commit_ish, str, None], check: bool =
         contain the ``.gitmodules`` blob.
 
         :param commit:
-            Commit-ish reference pointing at the root_tree, or ``None`` to always point
+            Commit-ish reference pointing at the root tree, or ``None`` to always point
             to the most recent commit.
 
         :param check: