spec: clarify implementation restrictions on untyped floats
spec: clarify implementation restrictions on untyped floats

Drop reference to "machine type."  Specify that integer
overflow must be an error.  Drop requirement that exponent
must be 128 bits--that's a lot.  Clarify that floating point
expressions may be rounded, including intermediate values.

This is a reworking of http://codereview.appspot.com/5577068/ .

Fixes issue 2789.

R=r, rsc, r, gri, ken, ken, iant
CC=golang-dev, remyoudompheng
http://codereview.appspot.com/5655049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go