Clarify Submodule.branch_path documentation

This changes "Full (relative) path" to "Complete relative path" in
the wording used to describe the branch_path initializer parameter
and property of the Submodule class. This is to prevent confusion,
since "full path" means something like "absolute path" (and is now
used as such in error messages introduced since these docstrings
were last edited).

git/objects/submodule/base.py

@@ -124,7 +124,7 @@ def __init__(
             the ``url`` parameter.
         :param parent_commit: See :meth:`set_parent_commit`.
         :param url: The URL to the remote repository which is the submodule.
-        :param branch_path: Full (relative) path to ref to checkout when cloning the
+        :param branch_path: Complete relative path to ref to checkout when cloning the
             remote repository.
         """
         super().__init__(repo, binsha, mode, path)
@@ -1322,7 +1322,7 @@ def branch(self) -> "Head":
     @property
     def branch_path(self) -> PathLike:
         """
-        :return: Full (relative) path as string to the branch we would checkout
+        :return: Complete relative path as string to the branch we would checkout
             from the remote and track
         """
         return self._branch_path