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

library/std/src/io/mod.rs

@@ -505,7 +505,8 @@ pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [
 /// [`std::io`]: self
 /// [`File`]: crate::fs::File
 #[stable(feature = "rust1", since = "1.0.0")]
-#[doc(spotlight)]
+#[cfg_attr(bootstrap, doc(spotlight))]
+#[cfg_attr(not(bootstrap), doc(notable_trait))]
 pub trait Read {
     /// Pull some bytes from this source into the specified buffer, returning
     /// how many bytes were read.
@@ -1291,7 +1292,8 @@ impl Initializer {
 ///
 /// [`write_all`]: Write::write_all
 #[stable(feature = "rust1", since = "1.0.0")]
-#[doc(spotlight)]
+#[cfg_attr(bootstrap, doc(spotlight))]
+#[cfg_attr(not(bootstrap), doc(notable_trait))]
 pub trait Write {
     /// Write a buffer into this writer, returning how many bytes were written.
     ///

library/std/src/lib.rs

@@ -257,7 +257,8 @@
 #![feature(doc_cfg)]
 #![feature(doc_keyword)]
 #![feature(doc_masked)]
-#![feature(doc_spotlight)]
+#![cfg_attr(bootstrap, feature(doc_spotlight))]
+#![cfg_attr(not(bootstrap), feature(doc_notable_trait))]
 #![feature(dropck_eyepatch)]
 #![feature(duration_constants)]
 #![feature(duration_zero)]