Re: Add A Glossary

I gave this a look.  I first reformatted it so I could read it; that's
0001.  Second I changed all the long <link> items into <xref>s, which

are shorter and don't have to repeat the title of the refered to page.
(Of course, this changes the link to be in the same style as every other
link in our documentation; some people don't like it. But it's our
style.)

There are some mistakes.  "Tupple" is most glaring one -- not just the
typo but also the fact that it goes to sql-revoke.  A few definitions
we'll want to modify.  Nothing too big.  In general I like this work and
I think we should have it in pg13.

Please bikeshed the definition of your favorite term, and suggest what
other terms to add.  No pointing out of mere typos yet, please.

I think we should have the terms Consistency, Isolation, Durability.