Tutorial: comment on how mutability applies to boxes
Tutorial: fix typo
Tutorial: spelling correction and move a failing test to an xfail-test marked code block.
auto merge of #5721 : dhardy/rust/master, r=graydon

This is some stuff which seemed to be missing to me (though I haven't read everything yet so hope I haven't missed the relevant section).

A similar addition for borrowing handles is needed, but #5720 stumped me.

Comments welcome.