mention phrase_to_file/3 which is being prepared in #1100

src/lib/format.pl

@@ -57,7 +57,7 @@
    If at all possible, format_//2 should be used, to stress pure parts
    that enable easy testing etc. If necessary, you can emit the list Ls
    with maplist(put_char, Ls) or, much faster, with format("~s", [Ls]).
-   Ideally, however, you use phrase_to_file/2 or phrase_to_stream/2
+   Ideally, however, you use phrase_to_file/[2,3] or phrase_to_stream/2
    from library(pio) to write the described list directly to a file
    or stream, respectively: phrase_to_stream(format_(..., [...]), S).
    The advantage of this is that an ideal implementation writes