Clarify VERTEX and other shader built-ins

[tetrapod00]

Closes #10250 and closes #7927. Note that this PR resolves half of #7927, with the other half, an explanation of local coordinates, now tracked in #9780.

The description for VERTEX in fragment() is longer than the corresponding descriptions for NORMAL, BINORMAL, and TANGENT, briefly describing how values are interpolated. While this is basic shader knowledge, it actually seems to be under-explained in the Godot docs in other places I would expect to see it. Since using VERTEX is so common, and since the name is somewhat confusing, I think it's okay to include such an explanatory note here.

I also added notes like "If skip_vertex_transform is enabled, it may not be in view space." to several other built-ins. As we add more and more built-ins, the table gets longer, and I don't think it's expected that users read the whole page to understand each description. I think we should start treating the built-in descriptions as closer to class member descriptions - they are expected to be read individually, with the optional context of the rest of the page. In this case, including a mention of skip_vertex_transform will prompt a reader to read the rest of the page, if applicable.

In the future we should consider changing these pages to something more like the class ref, with slightly longer descriptions, and copyable links to each individual shader built-in variable.

[mhilbrunner]

Thanks!