[RFC] extended txid docs

Although the new txid functions are very clean 1:1 interface
to the internal MVCC info and they don't need much docs
in that respect, their "killer" usage comes from the
possibility to query txids committed between 2 snapshots.

But how to do that (efficiently) is far from obvious when
just looking at the API.

So with attached docs patch I try to fill the gap. Here I
also show 2 variants for the common query helper function.

But I'm pretty bad at SGML, english and writing docs, so
please review it. In addition to english/typos/sgml
the suspicious aspects are:

- code style
- writing style
- used mostly PgQ terminology (ticks), could there be
something better?
- giving two variants of helper function may be too much

Even the realistic code may be too much for general docs,
but considering this is not a functionality covered
by general SQL textbooks, I think it is worth having.

I also put rendered pages up here:

http://skytools.projects.postgresql.org/txid/datatype-txid-snapshot.html
http://skytools.projects.postgresql.org/txid/functions-txid.html

--
marko