[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