Rollup merge of #54207 - QuietMisdreavus:never-docs-stab, r=kennytm

re-mark the never docs as unstable

Fixes https://github.com/rust-lang/rust/issues/54198

This stability attribute was removed in https://github.com/rust-lang/rust/pull/47630, but not replaced with a `#[stable]` attribute, and when https://github.com/rust-lang/rust/pull/50121 reverted that stabilization, it didn't set the docs back to unstable. I'm concerned as to why it was allowed to not have the stability attribute at all, but at least this can put it back.

I'm nominating this for beta backport because it's a really small change, and right now our docs are in an awkward position where the `!` type is technically unstable to use, but the docs don't say so the same way any other library feature would. (And this is also the case *on stable* now, but i'm not suggesting a stable backport for a docs fix.)

src/libstd/primitive_docs.rs

@@ -250,6 +250,7 @@ mod prim_bool { }
 /// [`Default`]: default/trait.Default.html
 /// [`default()`]: default/trait.Default.html#tymethod.default
 ///
+#[unstable(feature = "never_type", issue = "35121")]
 mod prim_never { }
 
 #[doc(primitive = "char")]