If you have a 1099r that you have to add to your amended return. what is an explaination that you can use in section III, that the irs will approve?

If you are amending to add a document you left out,

just say

"amending to include a document I left out."

Name the document type and amount,

be sure to include all the other forms or schedules that changed due to your "adding the document", and, if it shows federal tax withheld, a copy of the document.
If you are using the new e-File option, that task should be handled for you.