Rename `doc(spotlight)` to `doc(notable_trait)`

[camelid]

Leaving this as a todo for myself:

We should probably rename this flag to notable_trait since (a) it's clearer and (b) that's what the popup that it triggers is called. We should probably do it in another PR though; I'll try to do that soon.

Originally posted by @camelid in #80914 (comment)

@rustbot claim

[jyn514]

This is a user-facing change. I know doc(spotlight) is nightly only, but is the improvement worth it? I think spotlight is reasonably clear.

cc @Manishearth

[camelid]

I don't think it's that clear of a name. "spotlight" could mean many things, but doc(spotlight) means a very specific thing. For example, I could imagine "spotlight" meaning that rustdoc puts it at the top of the module page and highlights it.

[Manishearth]

Yeah, I feel like we should call it notable_trait or something since most users won't know the name "spotlight".