[docs] Add the description about mixing use of clang modules and c++

modules

From the discussion in
https://discourse.llvm.org/t/how-should-we-support-dependency-scanner-for-c-20-modules/66027,
we get a consensus that we want to support clang modules and c++ modules
at the same time. This patch documents this intention.

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D136221

Author: ChuanqiXu9

clang/docs/StandardCPlusPlusModules.rst

@@ -874,3 +874,10 @@ purposes of optimization (but definitions of these functions are still not inclu
 this means the build speedup at higher optimization levels may be lower than expected given ``O0`` experience, 
 but does provide by more optimization opportunities.
 
+Interoperability with Clang Modules
+-----------------------------------
+
+We **wish** to support clang modules and standard c++ modules at the same time,
+but the mixed using form is not well used/tested yet.
+
+Please file new github issues as you find interoperability problems.