docs: structs: more info on default field values

doc/docs.md

@@ -656,11 +656,15 @@ button.widget.set_pos(x,y)
 ```v
 struct Foo {
     n   int      // n is 0 by default
+    s   string   // s is '' by default
+    a   []int    // a is `[]int{}` by default
     pos int = -1 // custom default value
 }
 ```
 
-All struct fields are zeroed by default during the creation of the struct. But it's also possible to define custom default values.
+All struct fields are zeroed by default during the creation of the struct. Array and map fields are allocated. 
+
+It's also possible to define custom default values.
 
 ## Access modifiers