API Docs: fs

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/fs/

[steveklabnik]

#29300 is part of this

[steveklabnik]

Since File implements Seek, it should define what happens when you seek past the end. /cc https://www.reddit.com/r/rust/comments/4oh6vr/is_this_a_mistake_in_rust_document/d4cydzk