Emphasize relationship of depends_on and NXtransformations in NXsample

Add retangular beam profile and enumerate profiles

applications/NXmx.nxdl.xml

@@ -505,13 +505,34 @@
 				<field name="depends_on" type="NX_CHAR" minOccurs="0">
 					<!-- better type for paths the need to resolve -->
 					<doc>
-						This should be an absolute requirement to have for any scan
-						experiment.
+						This is a requirement to describe for any scan experiment.
 						The reason it is optional is mainly to accommodate XFEL
 						single shot exposures.
+
+						The axis on which the sample position depends may be stored
+						anywhere, but is normally stored in the NXtransformations
+						NXtransformations group within the NXsample group.
 					</doc>
 				</field>
 
+				<group type="NXtransformations" minOccurs="0">
+					<doc>
+						This is the recommended location for sample goniometer
+						and other related axes.
+
+						This is a requirement to describe for any scan experiment.
+						The reason it is optional is mainly to accommodate XFEL
+						single shot exposures.
+
+						Use of the depends_on field and the NXtransformations group is
+						strongly recommended.  As noted above this should be an absolute 
+						requirement to have for any scan experiment.
+
+						The reason it is optional is mainly to accommodate XFEL
+						single shot exposures.
+					</doc>
+				</group>
+
 				<field name="temperature" units="NX_TEMPERATURE" minOccurs="0" />
 
 				<group type="NXbeam">
@@ -562,15 +583,24 @@
 					<field name="incident_beam_size" type="NX_FLOAT" units="NX_LENGTH"  minOccurs="0">
 						<doc>
 							Two-element array of FWHM (if Gaussian or Airy function) or
-							diameters (if top hat) of beam in the order x, y
+							diameters (if top hat) or widths (if rectangular) of beam 
+							in the order x, y
 						</doc>
 						<dimensions rank="1" value="2" />
 					</field>
 
 					<field name="profile" type="NX_CHAR" minOccurs="0">
 						<doc>
-							The beam profile, such as Gaussian, Airy function, or top-hat.
+							The beam profile, Gaussian, Airy function, top-hat or
+							rectangular.  The profile is given in the plane of
+							incidence of the beam on the sample.
 						</doc>
+						<enumeration>
+							<item value="Gaussian"/>
+							<item value="Airy"/>
+							<item value="top-hat"/>
+							<item value="rectangular"/>
+						</enumeration>
 					</field>
 
 
@@ -581,12 +611,6 @@
 						</dimensions>
 					</field>
 				</group>
-
-				<group type="NXtransformations" minOccurs="0">
-					<doc>
-						Suggested location for sample goniometer or other axes (transformations)
-					</doc>
-				</group>
 			</group>
 
 			<group type="NXdata" />