doc/install: tweak port text; drop redundant list of operating systems.
doc/install: tweak port text; drop redundant list of operating systems.

R=adg
CC=golang-dev, r
http://codereview.appspot.com/597041

ssrl/go,bdwalton/debian-golang,h8liu/golang,gnanderson/go,h8liu/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,bdwalton/debian-golang,dsriram/golang,gnanderson/go,nightlyone/golang,tianon/debian-golang,h8liu/golang,h8liu/golang,pombredanne/go-deleteme,ssrl/go,nightlyone/golang,pombredanne/go-deleteme,ssrl/go,h8liu/golang,h8liu/golang,ssrl/go,dsriram/golang,bdwalton/debian-golang,bdwalton/debian-golang,tianon/debian-golang,nightlyone/golang,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,h8liu/golang,tv42/old-go,tianon/debian-golang,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,bdwalton/debian-golang,bdwalton/debian-golang,bdwalton/debian-golang,ssrl/go,nightlyone/golang,ssrl/go,apparentlymart/golang,dsriram/golang,gnanderson/go,tv42/old-go,tianon/debian-golang,tianon/debian-golang,dsriram/golang,nightlyone/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,tianon/debian-golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,nightlyone/golang,apparentlymart/golang,tianon/debian-golang,bdwalton/debian-golang,nightlyone/golang,tianon/debian-golang,ssrl/go