improve docs (#2068)

* improve decl_storage instance doc

* use decl_event doc accordignly

* automate doc and while allow to extend it on event

* some missed ones

* Update srml/example/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Update srml/support/procedural/src/storage/transformation.rs

substrate/node-template/runtime/src/template.rs

@@ -55,7 +55,6 @@ decl_module! {
 }
 
 decl_event!(
-	/// An event in this module.
 	pub enum Event<T> where AccountId = <T as system::Trait>::AccountId {
 		// Just a dummy event.
 		// Event `Something` is declared with a parameter of the type `u32` and `AccountId`