drm/i915/params: document I915_PARAMS_FOR_EACH()

Macros with this much magic in them deserve some explanatory text.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6f012851a54433b23cb4752f9d4ef523165b1e58.1545920737.git.jani.nikula@intel.com