Communicate the use of Storage Access Framework's fileURI more clearly

**Your Godot version:** 4.6 stable

**Issue description**

There are certain limitations when it comes to the new fileURI. It doesn't seem to have any support for `DirAccess` whatsoever. Meanwhile, `make_dir` and friends are what's mostly needed, since normally creating a file fails if any of the subdirectories does not exist. This isn't the case for Android's implementation of `FileAccess.open`, since I can successfully create files, and it will makedirs for me.

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/stable/classes/class_displayserver.html#class-displayserver-method-file-dialog-show

However, the specific place to clarify this is unclear. Also, what's there should be mirrored in file_dialog_with_options_show as well.

It is unclear whether `DirAccess` functions are intentionally not implemented, or if there was an oversight.