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/ConfigFile.xml

@@ -16,7 +16,7 @@
 		[codeblock]
 		var config = ConfigFile.new()
 		var err = config.load("user://settings.cfg")
-		if err == OK: # if not, something went wrong with the file loading
+		if err == OK: # If not, something went wrong with the file loading
 		    # Look for the display/width pair, and default to 1024 if missing
 		    var screen_width = config.get_value("display", "width", 1024)
 		    # Store a variable if and only if it hasn't been defined yet