Update tol.xml abs in mm

adjust note in man page stating that absolute tolerance is specified in model units (testing shows it is specified in mm)

doc/docbook/system/mann/tol.xml

@@ -38,7 +38,7 @@
   </para>
 <itemizedlist mark="bullet">
 <listitem>
-<para>abs -- This <emphasis>absolute</emphasis> tolerance is specified in model units and represents
+<para>abs -- This <emphasis>absolute</emphasis> tolerance is specified in mm and represents
 	the maximum allowable error in the distance from the actual shape surface
 	to the tessellated surface.  An <emphasis>absolute</emphasis> tolerance of 0 means that the
 	<emphasis>absolute</emphasis> tolerance should be ignored.