docs: Add docs for BUrlProtocolAsynchronousListener

Change-Id: Ib9df07d42d0153a7e0135acae85c00f3a1e6bd93
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2930
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

docs/user/net/UrlProtocolAsynchronousListener.dox

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2020 Haiku, Inc. All rights reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Leorize, leorize+oss@disroot.org
+ *
+ * Corresponds to:
+ *		headers/os/net/UrlProtocolAsynchronousListener.h hrev54280
+ *		src/kits/network/libnetapi/UrlProtocolAsynchronousListener.cpp hrev54280
+ */
+
+
+/*!
+	\file UrlProtocolAsynchronousListener.h
+	\ingroup network
+	\brief Provides the BUrlProtocolAsynchronousListener interface.
+*/
+
+
+/*!
+	\class BUrlProtocolAsynchronousListener
+	\ingroup network
+	\brief Provides a handler for BUrlProtocolDispatchingListener.
+
+	BUrlProtocolAsynchronousListener transparently handles BMessage issued by
+	BUrlProtocolDispatchingListener via callbacks from the BUrlProtocolListener
+	interface.
+*/
+
+
+/*!
+	\fn BUrlProtocolAsynchronousListener::BUrlProtocolAsynchronousListener(bool transparent = false)
+	\brief Create a BUrlProtocolAsynchronousListener.
+
+	This constructor will also add the created object to the list of be_app
+	handlers.
+
+	\param transparent Whether a BUrlProtocolListener object should be created
+		in conjunction with the current object. This object can be accessed via
+		SynchronousListener().
+*/
+
+/*!
+	\fn virtual BUrlProtocolAsynchronousListener::~BUrlProtocolAsynchronousListener()
+	\brief Default destructor for BUrlProtocolAsynchronousListener.
+
+	This destructor will free resources associated with the current object.
+*/
+
+/*!
+	\fn BUrlProtocolListener* BUrlProtocolAsynchronousListener::SynchronousListener()
+	\brief The synchronous listener created from the current object.
+
+	\returns A pointer to a BUrlProtocolListener object, or \c NULL if
+		\c transparent was \c false on object creation and/or memory couldn't
+		be allocated for the object at the time of creation.
+
+	\warning The returned pointer lifetime is associated with the current object.
+		Do not attempt to delete this pointer.
+*/
+
+/*!
+	\fn virtual void BUrlProtocolAsynchronousListener::MessageReceived(BMessage* message)
+	\brief Handle messages that has been received by the associated looper.
+
+	This handler handles messages received from
+	BUrlProtocolDispatchingListener and handle them via callbacks as defined per
+	BUrlProtocolListener.
+*/