Add info about discarding the three STD IO streams

Author: b2gills

doc/Type/Proc.pod6

@@ -68,7 +68,8 @@ C<$in>, C<$out> and C<$err> are the three
 standard streams of the to-be-launched program, and default to C<"-">, which
 means they inherit the stream from the parent process. Setting one (or more)
 of them to C<True> makes the stream available as an L<IO::Pipe> object of the
-same name, so for example C<$proc.out>. Or you can pass an existing
+same name, so for example C<$proc.out>. You can set them to C<False> to discard
+them. Or you can pass an existing
 L<IO::Handle> object (for example IO::Pipe) in, in which case this handle is
 used for the stream.