lib/alsa/AllowedFormat: add API documentation

src/lib/alsa/AllowedFormat.hxx

@@ -29,14 +29,28 @@ struct StringView;
 
 namespace Alsa {
 
+/**
+ * An audio format for the "allowed_formats" setting of
+ * #AlsaOutputPlugin.
+ */
 struct AllowedFormat {
 	AudioFormat format;
 #ifdef ENABLE_DSD
 	bool dop;
 #endif
 
+	/**
+	 * Parse a format string.
+	 *
+	 * Throws std::runtime_error on error.
+	 */
 	explicit AllowedFormat(StringView s);
 
+	/**
+	 * Parse a list of formats separated by space.
+	 *
+	 * Throws std::runtime_error on error.
+	 */
 	static std::forward_list<AllowedFormat> ParseList(StringView s);
 };