impl<T> is not introduced in the book

[shepmaster]

There's only a few references to impl< in TRPL:

1. src/doc/trpl/deref-coercions.md
2. src/doc/trpl/lifetimes.md
3. src/doc/trpl/unsized-types.md

None of these really mention that you use impl<T> to declare a typename that will participate in either the trait or item involved in the implementation.

[lastorset]

I've got a draft going in my fork. Will submit a PR soon, but feel free to review it even before I do.

[shepmaster]

@lastorset your docs look great! I added some initial comments in your branch. Thanks for taking a crack at this!

[steveklabnik]

@lastorset yeah, please send a PR whenever you're ready :)

[lastorset]

Thanks, I appreciate the feedback! I will work through my changes one more time before I submit.