Set ZAxisMapping default to "4 5 6 7".

README.sgml

@@ -485,7 +485,8 @@ Moreover, there are some mice whose two wheels are both mounted vertically,
 and the direction of the second vertical wheel does not match the
 first one's.
 
-You need to edit the <tt>xorg.conf</tt> file by hand to add this option.
+You need to edit the <tt>xorg.conf</tt> file by hand to change this option if 
+the default value of "4 5 6 7" does not match the needs of your configuration.
 
 <sect1>Resolution <p>
 The following option will set the mouse device resolution to <tt>N</tt>

man/mouse.man

@@ -161,7 +161,7 @@ of the second wheel mapped respectively to buttons number
 .I N3
 and
 .IR N4 .
-Default: no mapping.
+Default: "4 5 6 7".
 .TP 7
 .BI "Option \*qFlipXY\*q \*q" boolean \*q
 Enable/disable swapping the X and Y axes.  This transformation is applied

src/mouse.c

@@ -535,7 +535,7 @@ MouseCommonOptions(InputInfoPtr pInfo)
 	}
     }
 
-    s = xf86SetStrOption(pInfo->options, "ZAxisMapping", NULL);
+    s = xf86SetStrOption(pInfo->options, "ZAxisMapping", "4 5 6 7");
     if (s) {
 	int b1 = 0, b2 = 0, b3 = 0, b4 = 0;
 	char *msg = NULL;