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/InputEventMouseMotion.xml

@@ -13,10 +13,10 @@
 	</methods>
 	<members>
 		<member name="relative" type="Vector2" setter="set_relative" getter="get_relative">
-			Mouse position relative to the previous position (position at the last frame).
+			The mouse position relative to the previous position (position at the last frame).
 		</member>
 		<member name="speed" type="Vector2" setter="set_speed" getter="get_speed">
-			Mouse speed.
+			The mouse speed in pixels per second.
 		</member>
 	</members>
 	<constants>