Cleaned up _pia.h register stuff a bit

Cleaned up _pia.h register stuff a bit

Most documentation say that most of the bits are normally set to 1 or 0,
so just mentioning that in the comments.

A.N.A.L.O.G. issue 59 (April 1988) "Bits & Pieces" column, "Atari Zucchini"
(https://www.atarimagazines.com/analog/issue59/bits_pieces.php) implies
that they are used for specific things, but it's not clear enough to be
useful (or specifically states "Too complex and not pertinent").

This is probably sufficient for most purposes; if any PIA / PACTL/PBCTL
experts pop up, they can tell us exactly how those 4 bits can be utilized.