Clarify suffix usage when importing resources with ResourceImporterScene

[emre0altan]

Your Godot version: 4.2.1-stable

Issue description: Suffix usage when importing resources with ResourceImporterScene is not clear

URL to the documentation page: https://docs.godotengine.org/en/stable/tutorials/assets_pipeline/importing_3d_scenes/node_type_customization.html#create-navigation-navmesh

Explained suffix on the page is not clear that it works with "$" (case sensitive), "-" (case insensitive) and "_" (case insensitive). This issue came up here: godotengine/godot#87377. I'll create a PR for this.

[skyace65]

Fixed by #8951.