[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.

Author: ChuanqiXu9

clang/docs/StandardCPlusPlusModules.rst

@@ -365,6 +365,10 @@ the above example could be rewritten into:
 
   $ clang++ -std=c++20 M.cppm --precompile -fmodule-file=M:interface_part=M-interface_part.pcm -fmodule-file=M:impl_part=M-impl_part.pcm -o M.pcm
 
+When there are multiple ``-fmodule-file=<module-name>=`` options for the same
+``<module-name>``, the last ``-fmodule-file=<module-name>=`` will override the previous
+``-fmodule-file=<module-name>=`` options.
+
 ``-fprebuilt-module-path`` is more convenient and ``-fmodule-file`` is faster since
 it saves time for file lookup.