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/TreeItem.xml

@@ -90,7 +90,7 @@
 			<argument index="0" name="column" type="int">
 			</argument>
 			<description>
-				Returns the column's cell mode. See [code]CELL_MODE_*[/code] constants.
+				Returns the column's cell mode.
 			</description>
 		</method>
 		<method name="get_children">
@@ -527,7 +527,7 @@
 			<argument index="1" name="text_align" type="int" enum="TreeItem.TextAlign">
 			</argument>
 			<description>
-				Sets the given column's text alignment. See [code]ALIGN_*[/code] constants.
+				Sets the given column's text alignment. See [enum TextAlign] for possible values.
 			</description>
 		</method>
 		<method name="set_tooltip">