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/EditorInterface.xml

@@ -146,7 +146,7 @@
 			<return type="int" enum="Error">
 			</return>
 			<description>
-				Saves the scene. Returns either OK or ERR_CANT_CREATE. See [@GlobalScope] constants.
+				Saves the scene. Returns either [code]OK[/code] or [code]ERR_CANT_CREATE[/code] (see [@GlobalScope] constants).
 			</description>
 		</method>
 		<method name="save_scene_as">