archive/zip: add docs on type of slashes in paths

Fixes issue 5307

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8676046
doc/go1.1.html, mime/multipart: document quoted-printable decoding

Fixes issue 5293

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8840043