null.4: Minor wording and formatting fixes

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

man4/null.4

@@ -27,25 +27,21 @@
 .SH NAME
 null, zero \- data sink
 .SH DESCRIPTION
-Data written to a
-.BR null
-or
-.B zero
-special file is discarded.
+Data written to the
+.IR /dev/null
+and
+.IR /dev/zero
+special files is discarded.
 .PP
-Reads from the
-.B null
-special file always return end of file (i.e.,
+Reads from
+.IR /dev/null
+always return end of file (i.e.,
 .BR read (2)
-returns 0), whereas
-reads from
-.B zero
+returns 0), whereas reads from
+.IR /dev/zero
 always return bytes containing zero (\(aq\e0\(aq characters).
 .LP
-.B null
-and
-.B zero
-are typically created by:
+These devices are typically created by:
 .RS
 .sp
 mknod \-m 666 /dev/null c 1 3