cmd/go: update mkalldocs.sh

Update mkalldocs.sh from the current master sources, replacing the old
mkdoc.sh.

Change-Id: I22faeb5db3376d7a2881dd220aa0d07c6bcb5186
Reviewed-on: https://go-review.googlesource.com/111096
Reviewed-by: Than McIntosh <thanm@google.com>
2 files changed