Fix incomplete description for ResourceLoader.list_directory()

[Wierdox]

Description of ResourceLoader.list_directory() is bad.

• Doesn't say it includes directories. The fact it just says "returning all resources contained within" implies it only includes files that are resources.
• The way it appends / at the end of directories isn't in common with DirAccess(it's just the folder name), which seems like an inconsistency... but regardless it should be mentioned.
• Doesn't say it's sorted alphabetically.
• Should probably refer to DirAccess as the standard way of interacting with the filesystem.

My edit should cover the bases.

[Wierdox]

Initially a two word clarification, I increased the scope of my PR since I realized I missed some stuff. Of what I did, I can see one arguing that the codeblock example or the DirAccess reference being a step too far. Guess I'll find out.