add to docs that steps must be in ascending order

schema/cpacs_schema.xsd

@@ -1051,10 +1051,11 @@ [email protected]
                                     is defined in a so-called <path>, which is componsed of a list of <step> elements.
                                     </ddue:para>
                                     <ddue:para>The control parameter values for each step are arbitrary floating point values. However, it is strongly recommended to use
-                                    values between -1. and +1., or between 0. and +1. (depending on the type of control surface). The smallest and the largest value implicitly
-                                    define the maximum deflection limits. It is mandatory, that the value &#8220;0.&#8221; is within the specified range, as this value is treated as
-                                    undeflected and used to specify a &#8220;clean&#8221; aircraft configuration (e.g. used in the clean aero performance map). It is recommended, but not
-                                    mandatory to specify a &lt;step&gt; with a &lt;controlParameter&gt; of 0. Consequently, no &lt;controlParameter&gt; must be used twice within
+                                    values between -1 and +1, or between 0 and +1 (depending on the type of control surface). The smallest and the largest value implicitly
+                                    define the maximum deflection limits. It is mandatory, that the value &#8220;0&#8221; is within the specified range, as this value is treated as
+                                    undeflected and used to specify a &#8220;clean&#8221; aircraft configuration (e.g. used in the clean aero performance map). 
+                                    Furthermore, it it is mandatory for the &lt;step&gt; elements to be sorted in ascending order of the control parameters. 
+                                    It is recommended, but not mandatory to specify a &lt;step&gt; with a &lt;controlParameter&gt; of 0. Consequently, no &lt;controlParameter&gt; must be used twice within
                                     a single &lt;path&gt; definition. Deflection values between two specified steps are handled by linear interpolation.</ddue:para>
                                     <ddue:para>The following example shows the usage of control parameters within a control surface deflection path definition:</ddue:para>
                                     <ddue:code language="XML" title="Control parameter example">