[Clang] [Docs] Document runtime config directory options (#66593)
In the clang user manual the build options `CLANG_CONFIG_FILE_USER_DIR`
and `CLANG_CONFIG_FILE_SYSTEM_DIR` are documented, but the run time
overrides `--config-user-dir` and `--config-system-dir` are not.
I have updated the manual to add these run time arguments.