Rollup merge of #110060 - WaffleLapkin:sync_refs, r=jyn514

Document that `&T` and `&mut T` are `Sync` if `T` is

Proof: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=fec8dc9ec36e873bf2962a2367d67045

Author: JohnTitor

library/core/src/primitive_docs.rs

@@ -1362,6 +1362,7 @@ mod prim_usize {}
 /// * [`Hash`]
 /// * [`ToSocketAddrs`]
 /// * [`Send`] \(`&T` references also require <code>T: [Sync]</code>)
+/// * [`Sync`]
 ///
 /// [`std::fmt`]: fmt
 /// [`Hash`]: hash::Hash

library/std/src/primitive_docs.rs

@@ -1362,6 +1362,7 @@ mod prim_usize {}
 /// * [`Hash`]
 /// * [`ToSocketAddrs`]
 /// * [`Send`] \(`&T` references also require <code>T: [Sync]</code>)
+/// * [`Sync`]
 ///
 /// [`std::fmt`]: fmt
 /// [`Hash`]: hash::Hash