Closes #1322

Add an Enforcement rule for prefix inc/dec functions. Try to phrase it
to reduce noisiness.

CppCoreGuidelines.md

@@ -1069,6 +1069,9 @@ After that, we can look at waste related to algorithms and requirements, but tha
 
 Many more specific rules aim at the overall goals of simplicity and elimination of gratuitous waste.
 
+* Flag an unused return value from a user-defined non-defaulted postfix `operator++` or `operator--` function. Prefer using the prefix form instead. (Note: "User-defined non-defaulted" is intended to reduce noise. Review this enforcement if it's still too noisy in practice.)
+
+
 ### <a name="Rp-mutable"></a>P.10: Prefer immutable data to mutable data
 
 ##### Reason