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 https://golang.org/cl/5577068/ .

Fixes #2789.

R=r, rsc, r, gri, ken, ken, iant
CC=golang-dev, remyoudompheng
https://golang.org/cl/5655049