VERTEX shader built-in description confusing

[tetrapod00]

Your Godot version:
4.4
Issue description:
This issue came from a help chat discussion, making this issue to track for myself.

VERTEX in fragment() assumes a bit of shader knowledge. The current description "Vertex position that comes from the vertex() function (default, in view space)" assumes that you already know that the pixel/fragment position is the interpolated vertex position. I'm not even sure if the docs currently document how values get interpolated like this. The table can't explain how everything in shaders works, of course, but this seems like it can be made more clear with only a few words.

Possible improvements:

• Improve the VERTEX description itself, and likely the vertex() VERTEX and maybe vertex() POSITION ones too, to avoid confusion when searching for "position" in the page.
• Check that interpolation between vertex() and fragment() is covered somewhere, maybe mention it briefly in this page.

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/latest/tutorials/shaders/shader_reference/spatial_shader.html