doc/Makefile: add 'all' rule to build all docs
doc/Makefile: add 'all' rule to build all docs

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5334044

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