Add note to doc

Author: AnaDenisa

pandas/core/frame.py

@@ -13328,6 +13328,11 @@ def isin(self, values: Series | DataFrame | Sequence | Mapping) -> DataFrame:
         Series.str.contains: Test if pattern or regex is contained within a
             string of a Series or Index.
 
+        Notes
+        -----
+            We use `__iter__` (and not `__contains__`) to iterate over values
+            when checking if it contains the elements in DataFrame.
+
         Examples
         --------
         >>> df = pd.DataFrame(