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.