cmake: follow-up zephyr_dt_preprocess() fixes

Follow-up fixes to 64c7f502
("cmake: extensions: fix zephyr_dt_preprocess() CPP handling") that
weren't urgent enough for the hotfix, or didn't get noticed:

- error out on missing CPP argument again (this regression was
  introduced in 64c7f502)
- use a UNIX manpage style argument arity format (formatting issues
  were present when the function was introduced)
- fix incorrect signature comment: the CPP arguments are the
  preprocessor and its arguments, not CMAKE_DTS_PREPROCESSOR
  (this was present when the function was introduced)

Fixes: 64c7f502
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>