cmd/git(docs[GitStashCmd.push]): complete docstring

why: The docstring had a TODO placeholder instead of proper documentation.
what:
- Add proper description for push method
- Document path, patch, and staged parameters
- Add link to git-stash documentation

Author: tony

src/libvcs/cmd/git.py

@@ -4829,9 +4829,18 @@ def push(
         check_returncode: bool | None = None,
         **kwargs: t.Any,
     ) -> str:
-        """Git stash update.
+        """Push changes to the stash.
 
-        TODO: Fill-in
+        Wraps `git stash push <https://git-scm.com/docs/git-stash>`_.
+
+        Parameters
+        ----------
+        path :
+            Limit stash to specific path(s).
+        patch :
+            Interactively select hunks to stash.
+        staged :
+            Stash only staged changes.
 
         Examples
         --------