gh-121977: Add tips for handling unhashable data (#122075)

Doc/library/statistics.rst

@@ -485,6 +485,12 @@ However, for reading convenience, most of the examples show sorted sequences.
       >>> mode(["red", "blue", "blue", "red", "green", "red", "red"])
       'red'
 
+   Only hashable inputs are supported.  To handle type :class:`set`,
+   consider casting to :class:`frozenset`.  To handle type :class:`list`,
+   consider casting to :class:`tuple`.  For mixed or nested inputs, consider
+   using this slower quadratic algorithm that only depends on equality tests:
+   ``max(data, key=data.count)``.
+
    .. versionchanged:: 3.8
       Now handles multimodal datasets by returning the first mode encountered.
       Formerly, it raised :exc:`StatisticsError` when more than one mode was