math/big: clarify comment on lehmerGCD overflow

math/big: clarify comment on lehmerGCD overflow

A clarifying comment was added to indicate that overflow of a
single Word is not possible in the single digit calculation.
Lehmer's paper includes a proof of the bounds on the size of the
cosequences (u0, u1, u2, v0, v1, v2).

Change-Id: I98127a07aa8f8fe44814b74b2bc6ff720805194b
Reviewed-on: https://go-review.googlesource.com/77451
Reviewed-by: Robert Griesemer <gri@golang.org>