Stabilize field init shorthand

Closes #37340.

src/doc/book/src/structs.md

@@ -122,8 +122,6 @@ fields of the data structure are initialized with variables of the same
 names as the fields.
 
 ```
-#![feature(field_init_shorthand)]
-
 #[derive(Debug)]
 struct Person<'a> {
     name: &'a str,

src/doc/reference.md

@@ -2825,7 +2825,6 @@ This allows a compact syntax with less duplication.
 Example:
 
 ```
-# #![feature(field_init_shorthand)]
 # struct Point3d { x: i32, y: i32, z: i32 }
 # let x = 0;
 # let y_value = 0;