printf.3: SUSv3 and later agree with C99 for the snprintf() return value

Determined by inspection of the SUSv3 and SUSv4 specifications.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/printf.3

@@ -832,6 +832,9 @@ while C99 allows
 to be NULL in this case, and gives the return value (as always)
 as the number of characters that would have been written in case
 the output string has been large enough.
+SUSv3 and later align their specification of
+.BR snprintf ()
+with C99.
 .PP
 Linux libc4 knows about the five C standard flags.
 It knows about the length modifiers \fBh\fP, \fBl\fP, \fBL\fP,