docs: Missing Generics section.

[toqueteos]

Most "generic" mentions on manual are for "generic functions", a new section, similar to the one found on the tutorial would be a nice addition.

There are more than generic functions and traits, e.g:

struct Stack<T> {
    elements: ~[T]
}

enum Option<T> {
    Some(T),
    None
}

That new section would be a nice place to write a grammar for generic declarations, which would be definitely helpful for #9022.

[steveklabnik]

I am pretty sure this can be closed by now.