Update tutorial: 1-tuples now exist
Update manual for single-element tuples
Update manual for single-element tuple types
auto merge of #5782 : zofrex/rust/doc-1-tuples, r=thestinger

As per [the 0.6 release notes](https://github.com/mozilla/rust/wiki/Doc-detailed-release-notes#06-april-2013) single-element tuples are legal. I spotted a couple of places in the documentation that said otherwise, and propose these changes to them.