Fix signed/unsigned integer handling in pg_restore_relation_stats()

The pg_class fields relpages, relallvisible, and relallfrozen are of
type int32, but the statistics restoration code internally dealt with
them as uint32 for a little bit, after which they would turn back into
int32. This all happens to work, but it doesn't make sense, so I
propose the attached fix.

While researching this, I noticed that we don't appear to document how
page/block counts larger than INT32_MAX are represented in the catalogs.
A C programmer would have a certain expectation, but we shouldn't
expect everyone to guess that. So I'm proposing a small addition to the
catalog documentation to clarify this.