Guide: patterns.

Fixes #4417.
Guide: iterators
doc: grammar fixes
auto merge of #16621 : tshepang/rust/grammar, r=steveklabnik
auto merge of #16257 : steveklabnik/rust/guide_patterns, r=brson

Fixes #4417.
auto merge of #16408 : steveklabnik/rust/guide_iterators, r=brson

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
30 minute intro re-write