[release-branch.go1] net/http/pprof: updated documentation (run an http server)

««« backport 4965beed4492
net/http/pprof: updated documentation (run an http server)

Added instructions for starting an http server
to the godoc header for this package.  With the old
instructions, the example "go tool pprof..." commands
wouldn't work unless you happen to be running an http
server on port 6060 in your application.

R=golang-dev, minux.ma, adg, giacomo.tartari
CC=golang-dev
https://golang.org/cl/6483049

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