Update comments for PGPROC/PGXACT split

Many comment references to PGPROC and MyProc should now refer to PGXACT and
MyPgXact. This patch attempts to cover all such cases. In some places, a
comment refers collectively to all the xid-related fields, which span both
structures. I variously changed those to refer to either or both structures
depending on how I judged the emphasis.

The PGXACT patch extracted the PGPROC struct formerly embedded in each
GlobalTransaction. I updated a few comments accordingly.

The procarray.c leading comment described the procarray as "unsorted", but it
is sorted by memory address; I removed that word.

I include two typo-only fixes on related but already-accurate lines.

Thanks,
nm