Clarify definition of "input positions" in lifetime elision RFC.

Explicitly note that lifetimes from the `impl` (and `trait`/`struct`)
are not considered "input positions" for the purposes of expanded `fn`
definitions.

Added a collection of examples illustrating this.

Drive-by: Addressed a review comment from @chris-morgan
[here](https://github.com/rust-lang/rfcs/pull/141#discussion_r14227369).