patch 8.2.0357: cannot delete a text property matching both id and type

Problem:    Cannot delete a text property matching both id and type. (Axel
            Forsman)
Solution:   Add the "both" argument.

runtime/doc/textprop.txt

@@ -231,6 +231,7 @@ prop_remove({props} [, {lnum} [, {lnum-end}]])
 		{props} is a dictionary with these fields:
 		   id		remove text properties with this ID
 		   type		remove text properties with this type name
+		   both		"id" and "type" must both match
 		   bufnr	use this buffer instead of the current one
 		   all		when TRUE remove all matching text properties,
 				not just the first one