strings: clarify Replacer's replacement order

strings: clarify Replacer's replacement order

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/109375
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>