math/big: minor cleanups

- comment fixes
- s/z/x/ in (*rat).Float64 to match convention for functions
  returning a non-*Rat
- minor test output tweaking

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8327044
doc/go1.1.html: document the state of gccgo
Fixes issue 5053.

R=adg, iant
CC=golang-dev
https://codereview.appspot.com/8337043