add missing return value (dotnet#3019)

Author: mairaw

xml/System.Windows.Automation/AndCondition.xml

@@ -81,7 +81,7 @@
       <Parameters />
       <Docs>
         <summary>Retrieves an array of the subconditions for this condition.</summary>
-        <returns>To be added.</returns>
+        <returns>An array of the subconditions for this condition.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[