SF: 1397711 Set docs conflated immutable and hashable

Doc/lib/libstdtypes.tex

@@ -1224,7 +1224,7 @@ Notes:
 	    \label{types-set}}
 \obindex{set}
 
-A \dfn{set} object is an unordered collection of immutable values.
+A \dfn{set} object is an unordered collection of distinct hashable objects.
 Common uses include membership testing, removing duplicates from a sequence,
 and computing mathematical operations such as intersection, union, difference,
 and symmetric difference.