Rely on sizeof(typename) rather than sizeof(variable) in pqformat.h.

Rely on sizeof(typename) rather than sizeof(variable) in pqformat.h.

In each of the pq_writeintN functions, the three uses of sizeof() should
surely all be consistent.  I started out to make them all sizeof(ni),
but on reflection let's make them sizeof(typename) instead.  That's more
like our usual style elsewhere, and it's just barely possible that the
failures buildfarm member hornet has shown since 4c119fbcd went in are
caused by the compiler getting confused about sizeof() a parameter that
it's optimizing away.

In passing, improve a couple of comments.

Discussion: https://postgr.es/m/E1e2RML-0002do-Lc@gemulon.postgresql.org