Update Sales Order Processing Page to explain unique error surrounding back-to-back amendments

I have added the following language per a discussion and live testing with @msnow1 @jpetr and @csotomango. This error will be rare, but adding to the handbook for posterity.

"Note: In the case of multiple, back-to-back amendments/add-ons affecting the same subscription, the first amendment/add-on opportunity must be Closed Won (and the quote must be sent to Zuora) prior to the creation of an additional amendment/add-on. If an amendment/add-on is created prior to the closing of a previous amendment/add-on to the same subscription, Zuora will combine the delta amount for both quotes into the second amendment quote's subtotal, resulting in an incorrect order form."