Add explain struct field init shorthand

Author: Giang Nguyen

src/doc/reference.md

@@ -2757,6 +2757,19 @@ let base = Point3d {x: 1, y: 2, z: 3};
 Point3d {y: 0, z: 10, .. base};
 ```
 
+#### Struct field init shorthand
+
+When initializing a data structure (struct, enum, union) with named fields, allow writing `fieldname` as a shorthand for `fieldname: fieldname`. This allows a compact syntax for initialization, with less duplication.
+
+In the initializer for a `struct` with named fields, a `union` with named fields, or an enum variant with named fields, accept an identifier `field` as a shorthand for `field: field`.
+
+Example:
+
+```
+let a = SomeStruct { field1, field2: expression, field3 };
+let b = SomeStruct { field1: field1, field2: expression, field3: field3 };
+```
+
 ### Block expressions
 
 A _block expression_ is similar to a module in terms of the declarations that