docs for advisory locks

ok, here is the promised docs for the advisory locks.

some quick notes here: this is my first non trivial patch (albeit only
documentation) and i am a complete docbook novice. i also dont have
the capability to build docbook at the moment. so consider this a
very rough draft. any comments are welcome particularly from someone
willing to build the sgml and check for errors...sorry about that but
i wanted to get this up as soon as possible.

thanks
merlin