strings: document that order of pairs matters in NewReplacer
Update NewReplacer documentation to specify that in the case of
multiple matches at the same position, the matching old/new
pair that appears first in NewReplacer arguments takes precedence.
Fixes #32699
Change-Id: I9d0616d28e5cd8c9bfa301be201f2b0ebf361dff
Reviewed-on: https://go-review.googlesource.com/c/go/+/185099 Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Rob Pike <r@golang.org>