input/tidal: add API documentation

src/input/plugins/TidalTrackRequest.hxx

@@ -31,6 +31,11 @@
 
 class CurlRequest;
 
+/**
+ * Callback class for #TidalTrackRequest.
+ *
+ * Its methods must be thread-safe.
+ */
 class TidalTrackHandler
 	: public boost::intrusive::list_base_hook<boost::intrusive::link_mode<boost::intrusive::safe_link>>
 {
@@ -39,6 +44,9 @@ public:
 	virtual void OnTidalTrackError(std::exception_ptr error) noexcept = 0;
 };
 
+/**
+ * An asynchronous request for the streaming URL of a Tidal track.
+ */
 class TidalTrackRequest final : CurlResponseHandler {
 	CurlSlist request_headers;