NewReplacer's documentation says that "replacements are performed in
order", meaning that substrings are replaced in the order they appear
in the target string, and not that the old->new replacements are
applied in the order they're passed to NewReplacer.

Rephrase the doc to make this clearer.

Fixes #25071

Change-Id: Icf3aa6a9d459b94764c9d577e4a76ad8c04d158d
Reviewed-on: https://go-review.googlesource.com/109375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>