add description to gadget_complex_demo_form.html

demo/gadget_complex_demo_form.html

@@ -21,6 +21,15 @@
             Demo UI OLAP Query editor based on JSON Schema form generator
           </h3>
         </section>
+        <p>This form (Form)  consists of 2 interacted subforms (Rows and Columns).
+          Before Form filling the JSON schemas for
+          Columns and Rows are identical. After user chooses any data from any Dimension in one
+          subform (for example Rows) then JSON schema of the other subform (in example - Columns)
+          changes and as a result the selected
+          Dimension disappears in the the other subform (in example - Columns). So if user
+          chooses Year in Rows then all data from Dimension Time (Year, Month, Quarter, etc.)
+          disappears in Columns.</p>
+        <br>
         <div data-gadget-url="../jsonform.gadget.html"
              data-gadget-scope="columns"
              data-gadget-sandbox="public"></div>