Document the `DAA` instruction behavior

[Rangi42]

We already describe the nontrivial HALT instruction behavior, #1561 links to Pan Docs for STOP, we illustrate the BIT/RES/SET instructions, and clarify a few others. But DAA is just described as "Decimal Adjust Accumulator to get a correct BCD representation after an arithmetic instruction." Given how many online guides there are to this instruction on various platforms (1, 2, 3, 4, etc), there's clear demand for an explanation. It would also be a chance to note that the N and H flags aren't completely pointless.