fixup! [idioms/ctor] adds default constructor section

replaces fn main examples with rustdoc examples

idioms/ctor.md

@@ -6,6 +6,14 @@ Rust does not have constructors as a language construct. Instead, the
 convention is to use an associated `new` function to create an object:
 
 ```rust
+/// Time in seconds.
+///
+/// # Example
+///
+/// ```
+/// let s = Second::new(42);
+/// assert_eq!(42, s.value());
+/// ```
 pub struct Second {
     value: u64
 }
@@ -15,17 +23,21 @@ impl Second {
         Self { value }
     }
 }
-
-fn main() {
-    let s = Second::new(42);
-}
 ```
 
 ## Default Constructors
 
 Rust supports default constructors with the [`Default`][std-default] trait:
 
 ```rust
+/// Time in seconds.
+///
+/// # Example
+///
+/// ```
+/// let s = Second::default();
+/// assert_eq!(0, s.value());
+/// ```
 pub struct Second {
     value: u64
 }
@@ -35,26 +47,24 @@ impl Default for Second {
         Self { value: 0 }
     }
 }
-
-fn main() {
-    let default = Second::default();
-    assert_eq!(default.value, 0);
-}
 ```
 
 `Default` can also be derived if all types of all fields implement `Default`,
 like they do with `Second`:
 
 ```rust
+/// Time in seconds.
+///
+/// # Example
+///
+/// ```
+/// let s = Second::default();
+/// assert_eq!(0, s.value());
+/// ```
 #[derive(Default)]
 pub struct Second {
     value: u64
 }
-
-fn main() {
-    let default = Second::default();
-    assert_eq!(default.value, 0);
-}
 ```
 
 **Note:** When implementing `Default` for a type, it is neither required nor