[docs] [C++20] [Modules] Mark the compatiblity issue within clang-cl.exe

See
https://discourse.llvm.org/t/clang-cl-exe-support-for-c-modules/72257
for details. Simply, the user of clang-cl.exe can't get a good
experience with C++20 modules now. While we don't have resources to
support it well now, we should make it clear in the documentation.