Improving the GiST docs

I notice that the current GiST docs are pretty crazy. Here's my first
attempt at actually making them useful.

(http://developer.postgresql.org/docs/postgres/gist.html)

Interestingly enough, I had a chat with Joe Hellerstein while trying to
write this - he was interested in how GiST was going in PostgreSQL and
gave us permission to paraphrase his old GiST site for these docs.

I also had a bit of feedback from Oleg.

I'm sure I've made shocking errors all through this, but I just wanted
to get the ball rolling with respect to getting that awful GiST page
removed...

Comments...

Chris