[docs] Update StandardCPlusPlusModules.rst with clang18

Changed Things:
- Mentioning we need to specify BMIs for indirectly dependent BMIs too.
- Remove the note for `delayed-template-parsing` since
  https://github.com/llvm/llvm-project/issues/61068 got closed.
- Add a note for https://github.com/llvm/llvm-project/issues/78850 since
  we've seen it for a lot of times.
- Add a note for https://github.com/llvm/llvm-project/issues/78173 since
  we've seen it for a lot of times.