[interop][docs] Add compiler requirement line to user guide for reverse interop

Author: hyp

docs/CppInteroperability/UserGuide-CallingSwiftFromC++.md

@@ -25,6 +25,11 @@ A Swift library author might want to expose their interface to C++, to allow a C
 
 A Swift codebase is organized into units called modules. A module typically corresponds to a specific Xcode or Swift package manager target. Swift can generate a module interface file that presents a source view of the public Swift interface provided by the module. In addition to a Swift module interface, Swift can also generate a header file that contains C++ functions and classes that allow us to work with the Swift functions and types. We can import this header file into our C++ program to start using the Swift APIs from C++.
 
+### Compiler Requirements
+
+The header files generated by the Swift compiler can only be compiled by the Clang compiler, as
+the generated header relies on specific ABI attributes that are only supported by Clang.
+
 ### C++ Language And Library Requirements
 
 Importing Swift APIs into C++ requires certain C++ features introduced in newer C++ language standards. The following C++ standards are expected to work: