Improve the comment explaining how BCD works

- Added the alpha and beta index values into the explanation.
- Replaced the old usage of the Bits structure with BigInt.data
  as the accumulation sink.
- Show the fact that array bits outside of the relevant nibbles
  delineated by alpha and beta are DONT-CARE blocks by using 'x'
  characters instead of '0'. There is no need to waste writing to
  the DC blocks.
Add a note about the dont-care memory blocks