Docs: please mention panicking in vec! documentation

[e-oz]

Example of panic:
http://is.gd/tXpV3W

0 words "panic" here: https://doc.rust-lang.org/nightly/book/vectors.html
and here: https://doc.rust-lang.org/nightly/std/vec/

[steveklabnik]

This isn't due to Vec directly, it is due to Index.

That said, it should still be mentioned overall, but will certainly be part of my API docs work. Would you mind leaving a comment over on #29380? Thanks!