Documentation : Tutorial improvement...

Refactoring examples on implementation of generics for linked list.
Fixing typo of 'Note's for coherancy.

Adding internal links inside the tutorial example with traits,
generics etc...

    1349 // The followings examples directly calls the impl of Eq for list.

I am not really confident with "directly calls the impl of Eq for list." the
idea is to explain that you can compare with self.eq method and '==' operator
thanks to Eq impl.

Adding a note in 2.1 section.