mem.4: Minor wording and formatting fixes

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

man4/mem.4

@@ -27,20 +27,20 @@
 .SH NAME
 mem, kmem, port \- system memory, kernel memory and system ports
 .SH DESCRIPTION
-.B mem
+.IR /dev/mem
 is a character device file
 that is an image of the main memory of the computer.
 It may be used, for example, to examine (and even patch) the system.
 .LP
 Byte addresses in
-.B mem
+.IR /dev/mem
 are interpreted as physical memory addresses.
 References to nonexistent locations cause errors to be returned.
 .LP
 Examining and patching is likely to lead to unexpected results
 when read-only or write-only bits are present.
 .LP
-Since 2.6.26, and depending on the architecture, the
+Since Linux 2.6.26, and depending on the architecture, the
 .B CONFIG_STRICT_DEVMEM
 kernel configuration option limits the areas
 which can be accessed through this file.
@@ -56,12 +56,12 @@ chown root:kmem /dev/mem
 .RE
 .LP
 The file
-.B kmem
+.IR /dev/kmem
 is the same as
-.BR mem ,
+.IR /dev/mem ,
 except that the kernel virtual memory
 rather than physical memory is accessed.
-Since 2.6.26, this file is available only if the
+Since Linux 2.6.26, this file is available only if the
 .B CONFIG_DEVKMEM
 kernel configuration option is enabled.
 .LP
@@ -73,9 +73,9 @@ mknod \-m 640 /dev/kmem c 1 2
 chown root:kmem /dev/kmem
 .RE
 .LP
-.B port
+.IR /dev/port
 is similar to
-.BR mem ,
+.IR /dev/mem ,
 but the I/O ports are accessed.
 .LP
 It is typically created by: