net/http: better stand-alone FileServer doc example

Motivated by a deletion in the wiki, which had a better
example.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8288045
net/http: remove some obsolete TODOs

We've decided to leave logging to third-parties (there are too
many formats), which others have done.

And we can't change the behavior of the various response
fields at this point anyway. Plus I argue they're correct and
match their documention.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8391043