Add note about performance of typed containers from the interpreter.
Add note about performance of typed containers from the interpreter.

As title. Also makes a header for another section more clear.

Closes #6439
Closes #6435

seibert/numba,seibert/numba,gmarkall/numba,numba/numba,stuartarchibald/numba,stonebig/numba,stonebig/numba,gmarkall/numba,stuartarchibald/numba,stuartarchibald/numba,numba/numba,IntelLabs/numba,IntelLabs/numba,stuartarchibald/numba,cpcloud/numba,stonebig/numba,cpcloud/numba,stonebig/numba,stonebig/numba,cpcloud/numba,cpcloud/numba,numba/numba,gmarkall/numba,IntelLabs/numba,IntelLabs/numba,IntelLabs/numba,numba/numba,seibert/numba,numba/numba,seibert/numba,gmarkall/numba,seibert/numba,cpcloud/numba,stuartarchibald/numba,gmarkall/numba