Add more thorough description of unexpected specifications

Author: lcartey

cpp/common/src/codingstandards/cpp/exceptions/ExceptionFlow.qll

@@ -318,7 +318,14 @@ class FunctionCallThrowingExpr extends FunctionCall, ThrowingExpr {
       target = getTarget() and
       result = getAFunctionThrownType(target, _) and
       // [expect.spec] states that throwing an exception type that is prohibited
-      // by the specification will result in the program terminating. We therefore
+      // by the specification will result in the program terminating, unless
+      // a custom `unexpected_handler` is registered that throws an exception type
+      // which is compatible with the dynamic exception specification, or the
+      // dynamic exception specification lists `std::bad_exception`, in which case
+      // a `std::bad_exception` is thrown.
+      // As dynamic exception specifications and the `unexpected_handler` are both
+      // deprecated in C++14 and removed in C++17, we assume a default
+      // `std::unexpected` handler that calls `std::terminate` and therefore
       // do not propagate such exceptions to the call sites for the function.
       not (
         hasDynamicExceptionSpecification(target) and