document both audio module options

include/fggl/audio/null_audio.hpp

@@ -37,6 +37,7 @@ namespace fggl::audio {
 			void play(const AudioClipShort & /*clip*/, bool /*looping = false*/) override;
 	};
 
+    //! A dummy audio module that does nothing
 	struct NullAudio {
 		constexpr static const char *name = "fggl::audio::NULL";
 		constexpr static const std::array<modules::ServiceName, 1> provides = {

include/fggl/audio/openal/module.hpp

@@ -33,6 +33,7 @@ namespace fggl::audio {
 	constexpr auto OGG_VORBIS = assets::AssetType::make("audio/vorbis");
 	constexpr auto RES_OGG_VORBIS = assets::from_mime("audio/vorbis");
 
+    //! an audio module which uses openal(-soft) as a backend.
 	struct OpenAL {
 		constexpr static const char *name = "fggl::audio::OpenAL";
 		constexpr static const std::array<modules::ServiceName, 1> provides = {