docs: Add references to the database schema documentation.

docs: Add references to the database schema documentation.

I field lots of questions about "where's the documentation?"  Perhaps this
will help.

The changes to ovs-vsctl(8) add a couple of references to
ovs-vswitchd.conf.db(5) but they also rephrase a couple of paragraphs in
what seems to me an easier to understand style.

Signed-off-by: Ben Pfaff <blp@nicira.com>