Add documentation.

Activity/Observer.hpp

@@ -23,7 +23,10 @@ namespace Activity {
 */
 class Observer {
 	public:
+		/// Provides hints as to the sort of information presented on an LED.
 		enum LEDPresentation: uint8_t {
+			/// This LED informs the user of some sort of persistent state, e.g. scroll lock.
+			/// If this flag is absent then the LED describes an ephemeral state, such as media access.
 			Persistent = (1 << 0),
 		};