Tweak to #1516 per editors call

Author: hsutter

CppCoreGuidelines.md

@@ -20692,7 +20692,8 @@ Where desirable, they can be "instrumented" with additional functionality (e.g.,
 These Guidelines assume a `variant` type, but this is not currently in GSL.
 Eventually, use [the one voted into C++17](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0088r3.html).
 
-Some of the types listed below may not be supported in the library you use due to technical reasons such as limitations in the current versions of C++.
+These Guidelines use types from the standard (e.g., C++17) in addition to ones from the GSL.
+Some of the GSL types listed below may not be supported in the library you use due to technical reasons such as limitations in the current versions of C++.
 Therefore, please consult your GSL documentation to find out more.
 
 Summary of GSL components: