libobs: Add a couple missing comments

(Lain note: This is mostly so more (yes more) can be added to this
god-forsaken structure without it getting too messy. In terms of actual
code, I need to be better about writing actual code comments. ...Meaning
that I actually need to start writing code comments.)

libobs/obs-internal.h

@@ -870,9 +870,11 @@ struct obs_source {
 	/* color space */
 	gs_texrender_t *color_space_texrender;
 
+	/* audio monitoring */
 	struct audio_monitor *monitor;
 	enum obs_monitoring_type monitoring_type;
 
+	/* private data */
 	obs_data_t *private_settings;
 };