Better clarify Submodule.branch_path documentation

This revisits the changes in 3813bfb and takes a different
approach, using the phrase "full repository-relative path" that is
used elsewhere in the documentation and seems clear in context.

Although this brings back the potential confusion around having the
terms "full" and "relative" used together to describe a path, this
may no longer be an issue now that the phrase "repository-relative"
is used here as it is elsewhere.

(In particular, see the SymbolicReference.to_full_path docstring.)

Author: EliahKagan

git/objects/submodule/base.py

@@ -134,7 +134,7 @@ def __init__(
             The URL to the remote repository which is the submodule.
 
         :param branch_path:
-            Complete relative path to ref to checkout when cloning the remote
+            Full repository-relative path to ref to checkout when cloning the remote
             repository.
         """
         super().__init__(repo, binsha, mode, path)
@@ -1473,8 +1473,8 @@ def branch(self) -> "Head":
     def branch_path(self) -> PathLike:
         """
         :return:
-            Complete relative path as string to the branch we would checkout from the
-            remote and track
+            Full repository-relative path as string to the branch we would checkout from
+            the remote and track
         """
         return self._branch_path