[docs] [C++20] [Modules] Add document for the case of duplicated '-fmodule-file=<module-name>='

This patch clarifies that in case there are multiple
'-fmodule-file=<module-name>=' optins for the same `<module-name>`,
the last '-fmodule-file=<module-name>=' will override the previous
'-fmodule-file=<module-name>=' option.