Reformulate the paragraph about the axis show option slightly

git-svn-id: https://flot.googlecode.com/svn/trunk@312 1e0a6537-2640-0410-bfb7-f154510ff394

API.txt

@@ -205,9 +205,10 @@ All axes have the same kind of options. The following describes how to
 configure one axis, see below for what to do if you've got more than
 one x axis or y axis.
 
-The "show" option defaults to null which means the axis will show up
-if there's data associated with it. You can override this by setting
-"show" to true or false.
+If you don't set the "show" option (i.e. it is null), visibility is
+auto-detected, i.e. the axis will show up if there's data associated
+with it. You can override this by setting the "show" option to true or
+false.
 
 The "position" option specifies where the axis is placed, bottom or
 top for x axes, left or right for y axes. The "mode" option determines