Add docs for Missing, correct Option's Try test

src/libcore/option.rs

@@ -1124,7 +1124,7 @@ impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
     }
 }
 
-/// The `Option` type. See [the module level documentation](index.html) for more.
+/// The equivalent of `Option::None` for a `Result::Err`.
 #[unstable(feature = "try_trait", issue = "42327")]
 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 pub struct Missing;

src/libcore/tests/option.rs

@@ -286,13 +286,13 @@ fn test_try() {
     assert_eq!(try_option_none(), None);
 
     fn try_option_ok() -> Result<u8, Missing> {
-        let val = Ok(1)?;
+        let val = Some(1)?;
         Ok(val)
     }
     assert_eq!(try_option_ok(), Ok(1));
 
     fn try_option_err() -> Result<u8, Missing> {
-        let val = Err(Missing)?;
+        let val = None?;
         Ok(val)
     }
     assert_eq!(try_option_err(), Err(Missing));