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/EditorFileSystemDirectory.xml

@@ -97,7 +97,7 @@
 			<return type="EditorFileSystemDirectory">
 			</return>
 			<description>
-				Returns the parent directory for this directory or null if called on a directory at [code]res://[/code] or [code]user://[/code].
+				Returns the parent directory for this directory or [code]null[/code] if called on a directory at [code]res://[/code] or [code]user://[/code].
 			</description>
 		</method>
 		<method name="get_path" qualifiers="const">