Update EditorResourcePreview queue_*() documentation

(cherry picked from commit 72446bab1b1dd88c37aa29ea0140d8b413bb4683)

doc/classes/EditorResourcePreview.xml

@@ -40,7 +40,8 @@
 			<argument index="3" name="userdata" type="Variant">
 			</argument>
 			<description>
-				Queue a resource being edited for preview (using an instance). Once the preview is ready, your receiver.receiver_func will be called either containing the preview texture or an empty texture (if no preview was possible). Callback must have the format: (path,texture,userdata). Userdata can be anything.
+				Queue the [code]resource[/code] being edited for preview. Once the preview is ready, the [code]receiver[/code]'s [code]receiver_func[/code] will be called. The [code]receiver_func[/code] must take the following four arguments: [String] path, [Texture] preview, [Texture] thumbnail_preview, [Variant] userdata. [code]userdata[/code] can be anything, and will be returned when [code]receiver_func[/code] is called.
+				[b]Note[/b]: If it was not possible to create the preview the [code]receiver_func[/code] will still be called, but the preview will be null.
 			</description>
 		</method>
 		<method name="queue_resource_preview">
@@ -55,7 +56,8 @@
 			<argument index="3" name="userdata" type="Variant">
 			</argument>
 			<description>
-				Queue a resource file for preview (using a path). Once the preview is ready, your receiver.receiver_func will be called either containing the preview texture or an empty texture (if no preview was possible). Callback must have the format: (path,texture,userdata). Userdata can be anything.
+				Queue a resource file located at [code]path[/code] for preview. Once the preview is ready, the [code]receiver[/code]'s [code]receiver_func[/code] will be called. The [code]receiver_func[/code] must take the following four arguments: [String] path, [Texture] preview, [Texture] thumbnail_preview, [Variant] userdata. [code]userdata[/code] can be anything, and will be returned when [code]receiver_func[/code] is called.
+				[b]Note[/b]: If it was not possible to create the preview the [code]receiver_func[/code] will still be called, but the preview will be null.
 			</description>
 		</method>
 		<method name="remove_preview_generator">

editor/editor_resource_preview.h

@@ -103,7 +103,8 @@ protected:
 public:
 	static EditorResourcePreview *get_singleton();
 
-	//callback function is callback(String p_path,Ref<Texture> preview,Variant udata) preview null if could not load
+	// p_receiver_func callback has signature (String p_path, Ref<Texture> p_preview, Ref<Texture> p_preview_small, Variant p_userdata)
+	// p_preview will be null if there was an error
 	void queue_resource_preview(const String &p_path, Object *p_receiver, const StringName &p_receiver_func, const Variant &p_userdata);
 	void queue_edited_resource_preview(const Ref<Resource> &p_res, Object *p_receiver, const StringName &p_receiver_func, const Variant &p_userdata);