input/RemoteTagScanner: add API documentation

src/input/RemoteTagScanner.hxx

@@ -24,12 +24,35 @@
 
 struct Tag;
 
+/**
+ * Handler for the #RemoteTagScanner result.  It will call one of the
+ * methods upon completion.  Must be thread-safe.
+ */
 class RemoteTagHandler {
 public:
+	/**
+	 * Called on success.
+	 */
 	virtual void OnRemoteTag(Tag &&tag) noexcept = 0;
+
+	/**
+	 * Called on error.
+	 */
 	virtual void OnRemoteTagError(std::exception_ptr e) noexcept = 0;
 };
 
+/**
+ * This class can load tags of a remote file.  It is created by
+ * InputPlugin::scan_tags(), and the #RemoteTagHandler will be called
+ * upon completion.
+ *
+ * To start the operation, call Start().
+ *
+ * You can cancel the operation at any time by destructing this
+ * object; after successful cancellation, the handler will not be
+ * invoked, though it cannot be guaranteed that the handler is not
+ * already being called in another thread.
+ */
 class RemoteTagScanner {
 public:
 	virtual ~RemoteTagScanner() noexcept = default;