Proofread and improve the whole class reference

- Document a few more properties and methods
- Add more information to many classes
- Fix lots of typos and gramar mistakes
- Use [code] tags for parameters consistently
- Use [b] and [i] tags consistently
- Put "Warning:" and "Note:" on their own line to be more visible,
  and make them always bold
- Tweak formatting in code examples to be more readable
- Use double quotes consistently
- Add more links to third-party technologies

doc/classes/InputEventScreenDrag.xml

@@ -1,8 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="InputEventScreenDrag" inherits="InputEvent" category="Core" version="3.2">
 	<brief_description>
-		Input event type for screen drag events.
-		(only available on mobile devices)
+		Input event type for screen drag events. Only available on mobile devices.
 	</brief_description>
 	<description>
 		Contains screen drag information. See [method Node._input].
@@ -14,16 +13,16 @@
 	</methods>
 	<members>
 		<member name="index" type="int" setter="set_index" getter="get_index">
-			Drag event index in the case of a multi-drag event.
+			The drag event index in the case of a multi-drag event.
 		</member>
 		<member name="position" type="Vector2" setter="set_position" getter="get_position">
-			Drag position.
+			The drag position.
 		</member>
 		<member name="relative" type="Vector2" setter="set_relative" getter="get_relative">
-			Drag position relative to its start position.
+			The drag position relative to its start position.
 		</member>
 		<member name="speed" type="Vector2" setter="set_speed" getter="get_speed">
-			Drag speed.
+			The drag speed.
 		</member>
 	</members>
 	<constants>