Document the container types

[stuartarchibald]

https://numba.pydata.org/numba-doc/dev/reference/types.html doesn't have the container types documented. Now numba.objmode is implemented and adheres to the context manager protocol, it's feasible that users would want to define types that leave an objmode block that are more complex than those covered by the current docs (which I think are largely in relation to type signatures for {g,}ufuncs ans so relatively primitive).