DOC: add note on duplicate indices on parameter other (pandas-dev#55509)

Author: aureliobarbosa

pandas/core/frame.py

@@ -8692,6 +8692,10 @@ def update(
         dict.update : Similar method for dictionaries.
         DataFrame.merge : For column(s)-on-column(s) operations.
 
+        Notes
+        --------
+        1. Duplicate indices on `other` are not supported and raises `ValueError`.
+
         Examples
         --------
         >>> df = pd.DataFrame({"A": [1, 2, 3], "B": [400, 500, 600]})