Add validation when "declaring credit" for receiving batch row

i.e. don't just blindly attempt, when it isn't supported

tailbone/templates/receiving/declare_credit.mako

@@ -39,6 +39,22 @@
 <div style="display: flex; justify-content: space-between;">
 
   <div class="form-wrapper">
+
+    <p style="padding: 1em;">
+      Please select the "state" of the product, and enter the appropriate
+      quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Note that this tool will <strong>deduct</strong> from the "received"
+      quantity, and <strong>add</strong> to the corresponding credit quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Please see ${h.link_to("Receive Row", url('{}.receive_row'.format(route_prefix), uuid=batch.uuid, row_uuid=row.uuid))}
+      if you need to "receive" instead of "convert" the product.
+    </p>
+
     ${form.render()|n}
   </div><!-- form-wrapper -->
 

tailbone/templates/receiving/receive_row.mako

@@ -39,6 +39,22 @@
 <div style="display: flex; justify-content: space-between;">
 
   <div class="form-wrapper">
+
+    <p style="padding: 1em;">
+      Please select the "state" of the product, and enter the appropriate
+      quantity.
+    </p>
+
+    <p style="padding: 1em;">
+      Note that this tool will <strong>add</strong> the corresponding
+      quantities for the row.
+    </p>
+
+    <p style="padding: 1em;">
+      Please see ${h.link_to("Declare Credit", url('{}.declare_credit'.format(route_prefix), uuid=batch.uuid, row_uuid=row.uuid))}
+      if you need to "convert" some already-received amount, into a credit.
+    </p>
+
     ${form.render()|n}
   </div><!-- form-wrapper -->