[improve docs]: Timestamp pallet (#1435)

This PR improves the docs for the Timestamp pallet by following our
[Documentation
Guidelines](https://github.com/paritytech/polkadot-sdk/blob/master/docs/DOCUMENTATION_GUIDELINE.md)
more closely.

---------

Co-authored-by: Juan <juangirini@gmail.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

substrate/frame/timestamp/src/tests.rs

@@ -30,6 +30,7 @@ fn timestamp_works() {
 	});
 }
 
+#[docify::export]
 #[test]
 #[should_panic(expected = "Timestamp must be updated only once in the block")]
 fn double_timestamp_should_fail() {
@@ -39,6 +40,7 @@ fn double_timestamp_should_fail() {
 	});
 }
 
+#[docify::export]
 #[test]
 #[should_panic(
 	expected = "Timestamp must increment by at least <MinimumPeriod> between sequential blocks"