auto merge of #16119 : steveklabnik/rust/guide_pointers, r=brson

This is the next section of the guide, and it's on pointers. It's not done yet, as I need to write the section on ownership and borrowing, but I figured I'd share the rest now, to get feedback on the rest of it while I take some time to get that right.

diff --cc src/doc/guide.md
Simple merge