Revise documentation for new freezing method.

Commit 37484ad2aacef5ec794f4dd3d5cf814475180a78 invalidated a good
chunk of documentation, so patch it up to reflect the new state of
play.  Along the way, patch remaining documentation references to
FrozenXID to say instead FrozenTransactionId, so that they match the
way we actually spell it in the code.
Fix ANALYZE failure on a column that's a domain over a range.

Most other range operations seem to work all right on domains,
but this one not so much, at least not since commit 918eee0c.
Per bug #8684 from Brett Neumeier.