Fix some issues in pg_class.relminmxid and pg_database.datminmxid documentation.

Fix some issues in pg_class.relminmxid and pg_database.datminmxid documentation.

- Correct the name of directory which those catalog columns allow to be shrunk.
- Correct the name of symbol which is used as the value of pg_class.relminmxid
  when the relation is not a table.
- Fix "ID ID" typo.

Backpatch to 9.3 where those cataog columns were introduced.