Add a comment to the protocol specification about iterator key type.

(hat tip to Petr Yanovich).

doc/box-protocol.txt

@@ -178,7 +178,9 @@
 
 ; <offset> specifies offset in the result set, expects <uint32> value
 ; <limit> specifies limit in the result set, expects a <uint32> value
-; <iterator> specifies the iterator type to use in search
+; <iterator> specifies the iterator type to use in search,
+; an integer constant from the range defined in
+; https://github.com/tarantool/tarantool/blob/master/src/box/index.h#L61
 ; <function_name> is used to give call path for a Lua function
 
 ; <tuple> in <update> must carry a list of update operations: