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/EditorFileSystem.xml

@@ -15,14 +15,14 @@
 			<argument index="0" name="path" type="String">
 			</argument>
 			<description>
-				Get the type of the file, given the full path.
+				Gets the type of the file, given the full path.
 			</description>
 		</method>
 		<method name="get_filesystem">
 			<return type="EditorFileSystemDirectory">
 			</return>
 			<description>
-				Get the root directory object.
+				Gets the root directory object.
 			</description>
 		</method>
 		<method name="get_filesystem_path">