cmd/godoc: always include /doc files in union filesystems
cmd/godoc: always include /doc files in union filesystems

Makes Path rewrites work, as the metadata was never being scanned.

Fixes issue 3282.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5783076

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang