disallow pointer to pointer in api

docs/interop.md

@@ -7,6 +7,7 @@ for that certain considerations are taken:
   with no expectation on new additions (see [/include/twn_types.h](../include/twn_types.h))
 * optionals can be expressed via pointer passage of value primitives, assumed immutable, with the NULL expressing default
 * no opaque types, only keys
+* pointers to pointers aren't allowed
 * when mutation on input is done, - it shouldn't be achieved from a mutable pointer, but the return value
 * return value could be a simple aggregate that is translatable to pure data dictionary
 * module prefix is used for namespacing, actual symbols come after the prefix (`<module>_<symbol>`)