Document safe usage of undefined behavior in gsl::narrow (microsoft#1024)

Author: dmitrykobets-msft

include/gsl/narrow

@@ -36,7 +36,12 @@ GSL_SUPPRESS(f.6) // NO-FORMAT: attribute // TODO: MSVC /analyze does not recogn
     constexpr const bool is_different_signedness =
         (std::is_signed<T>::value != std::is_signed<U>::value);
 
-    const T t = narrow_cast<T>(u);
+GSL_SUPPRESS(es.103) // NO-FORMAT: attribute // don't overflow
+GSL_SUPPRESS(es.104) // NO-FORMAT: attribute // don't underflow
+GSL_SUPPRESS(p.2) // NO-FORMAT: attribute // don't rely on undefined behavior
+    const T t = narrow_cast<T>(u); // While this is technically undefined behavior in some cases (i.e., if the source value is of floating-point type
+                                   // and cannot fit into the destination integral type), the resultant behavior is benign on the platforms
+                                   // that we target (i.e., no hardware trap representations are hit).
 
     if (static_cast<U>(t) != u || (is_different_signedness && ((t < T{}) != (u < U{}))))
     {