Document pass-by-move arguments / modes in general

[catamorphism]

This email on rust-dev points out that move arguments (with a '-') are not documented in the tutorial or reference manual:

https://mail.mozilla.org/pipermail/rust-dev/2012-April/001632.html

They should be documented.

[catamorphism]

It's now explained in the tutorial, but should also be explained in the reference manual.

[graydon]

Modes are not described at all in the reference manual. I am not sure they should be; with any luck they'll be removed in 0.4 (which I hope we release quickly after 0.3).

[graydon]

We have consensus on modes going away and a plan in progress. Closing this.