[PATCH] Docs: Make notes on sequences and rollback more obvious

Hi all

I'm seeing enough questions on pgsql-general and stack overflow to
suggest that the docs for how sequences interact with transaction
rollback. Take the most recent post on -general, where the person read
at least the tutorial, but had no idea about the exemption.

The attached patch:

- Moves the note about nextval() from the footer to be inside the
nextval description

- Adds an xref from the advanced-transactions tutorial where the poster
noted their point of confusion, noting the exemption and pointing to the
docs on nextval.

- A pointer from the docs on SERIAL types to the nextval notes on tx
rollback.

Comments would be appreciated.

--
Craig Ringer