fixes #571

nxdl.xsd

@@ -1242,7 +1242,11 @@
 		<xs:attribute name="rank" type="nx:NX_CHAR">
 			<xs:annotation>
 				<xs:documentation>
-					Rank (number of dimensions) of the data structure.  
+					Rank (number of dimensions) of the data structure.
+
+					Value could be either an unsigned integer or a symbol
+					as defined in the *symbol* table of the NXDL file.
+
 					For example: ``a[5]`` has ``rank="1"`` while 
 					``b[8,5,6,4]`` has ``rank="4"``.  
 					See http://en.wikipedia.org/wiki/Rank_(computer_programming)