doc: add a struct reference example (#13638)

doc/docs.md

@@ -1929,6 +1929,32 @@ println(p.x)
 The type of `p` is `&Point`. It's a [reference](#references) to `Point`.
 References are similar to Go pointers and C++ references.
 
+```v
+struct Foo {
+mut:
+	x int
+}
+
+fa := Foo{1}
+mut a := fa
+a.x = 2
+assert fa.x == 1
+assert a.x == 2
+
+// fb := Foo{ 1 }
+// mut b := &fb  // error: `fb` is immutable, cannot have a mutable reference to it
+// b.x = 2
+
+mut fc := Foo{1}
+mut c := &fc
+c.x = 2
+assert fc.x == 2
+assert c.x == 2
+println(fc) // Foo{ x: 2 }
+println(c) // &Foo{ x: 2 } // Note `&` prefixed.
+```
+see also [Stack and Heap](#stack-and-heap)
+
 ### Default field values
 
 ```v