math/big: better documentation for Rat.Float64

R=adonovan, r
CC=golang-dev
https://codereview.appspot.com/11961043
runtime: fix arm build

The current failure is:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_si2v(0xb6ebaebc, 0x3b9aca00)
        /usr/local/go/src/pkg/runtime/vlrt_arm.c:628 fp=0xb6ebae9c
runtime.timediv(0xf8475800, 0xd, 0x3b9aca00, 0xb6ebaef4)
        /usr/local/go/src/pkg/runtime/runtime.c:424 +0x1c fp=0xb6ebaed4

Just adding textflag 7 causes the following error:
notetsleep: nosplit stack overflow
        128	assumed on entry to notetsleep
        96	after notetsleep uses 32
        60	after runtime.futexsleep uses 36
        4	after runtime.timediv uses 56
        -4	after _si2v uses 8

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12001045