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

drivers/gpu/drm/i915/i915_params.h

@@ -33,6 +33,15 @@ struct drm_printer;
 #define ENABLE_GUC_SUBMISSION		BIT(0)
 #define ENABLE_GUC_LOAD_HUC		BIT(1)
 
+/*
+ * Invoke param, a function-like macro, for each i915 param, with arguments:
+ *
+ * param(type, name, value)
+ *
+ * type: parameter type, one of {bool, int, unsigned int, char *}
+ * name: name of the parameter
+ * value: initial/default value of the parameter
+ */
 #define I915_PARAMS_FOR_EACH(param) \
 	param(char *, vbt_firmware, NULL) \
 	param(int, modeset, -1) \