ResourceLoader documentation should explain that all methods with res:// path parameter (exists, load, etc.) support passing UID

**Your Godot version:**
v4.6.stable.official [89cea1439]

**Issue description:**
I was hesitating a lot when using `ResourceLoader.exists` and `load` with my variable:

```gd
@export_file("*.tscn") var scene_uid: String
```

(which stores a UID whether toggling the Inspector view to show `res://` path or `uid://`, as explained in both `@export_file` doc and the view mode toggle button tooltip)

as I wasn't sure if it was supporting scene file UID or just asset path as written in the documentation. So I would tend to convert all my variables with `ResourceUID.path_to_uid` but most of the time, it wasn't necessary.

For instance, the doc for `exists` just says:

> ```gd
> bool exists(path: String, type_hint: String = "")
> ```
>
> Returns whether a recognized resource exists for the given path.

Looking at https://godotengine.org/article/uid-changes-coming-to-godot-4-4/ it seems that Godot calls both `res://` and `uid://` "paths", so maybe it is considered obvious that Godot methods are res path/UID-agnostic now, but I think it's worth indicating for people who've been working with older versions of Godot and wondering how the transition affected the resource loading API (in my mind, a UID was a number not a path).

To avoid repetition, maybe just mentioning once at the top that both res path and UID path are supported.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_resourceloader.html

Open the linked ResourceLoader class documentation and review the descriptions and signatures for exists, load, and other methods that accept a resource path. Add a concise explanation that these methods accept both res:// paths and uid:// paths, then verify the wording is consistent across the page and the documentation builds successfully.

Written by the indexing model from the issue text.