      archive/zip: Document ModTime is always UTC
      archive/zip: Document ModTime is always UTC

Fixes issue 7592

LGTM=robert.hencke, adg
R=golang-codereviews, robert.hencke, gobot, adg
CC=golang-codereviews
https://codereview.appspot.com/90810043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go