Don't mix future runtime customization constexpr.

As discussed in https://github.com/isocpp/CppCoreGuidelines/issues/420

CppCoreGuidelines.md

@@ -2079,6 +2079,13 @@ This is usually a very good thing.
 
 Don't try to make all functions `constexpr`. Most computation is best done at run time.
 
+##### Note
+
+Any API that may eventually become configurable or customizable at runtime
+should not be made `constexpr`. Such customization could not be evaluated by the
+compiler, and any `constexpr` functions that depend upon that API will have to
+be refactored or drop `constexpr`.
+
 ##### Enforcement
 
 Impossible and unnecessary.