Rename #[doc(spotlight)] to #[doc(notable_trait)]

"spotlight" is not a very specific or self-explaining name.
Additionally, the dialog that it triggers is called "Notable traits".
So, "notable trait" is a better name.

* Rename `#[doc(spotlight)]` to `#[doc(notable_trait)]`
* Rename `#![feature(doc_spotlight)]` to `#![feature(doc_notable_trait)]`
* Update documentation
* Improve documentation

compiler/rustc_span/src/symbol.rs

@@ -476,6 +476,7 @@ symbols! {
         doc_cfg,
         doc_keyword,
         doc_masked,
+        doc_notable_trait,
         doc_spotlight,
         doctest,
         document_private_items,
@@ -799,6 +800,7 @@ symbols! {
         noreturn,
         nostack,
         not,
+        notable_trait,
         note,
         object_safe_for_dispatch,
         of,