math/big: reintroduce pre-Go 1.14 mention in GCD docs

It was removed in CL 217302 but was intentionally added in CL 217104.

Change-Id: I1a478d80ad1ec4f0a0184bfebf8f1a5e352cfe8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/217941


Reviewed-by: Robert Griesemer <gri@golang.org>