clarify docstring

Author: darcymeyer

pandas/util/testing.py

@@ -1215,6 +1215,10 @@ def assert_frame_equal(left, right, check_dtype=True,
                        obj='DataFrame'):
     """Check that left and right DataFrame are equal.
 
+    Each element at [row, column] in left is compared to the
+    element at [row, column] in right (index and column values
+    may not be ignored).
+
     Parameters
     ----------
     left : DataFrame