[docs] [C++20] [Modules] Document how to import modules in clang-repl

We support to import C++20 named modules now in in clang-repl in
https://github.com/llvm/llvm-project/commit/dd3e6c87f3f4affd17d05a4d25fa77d224a98d94.

Then we should document how can we do that.