DOCS: added a section on database authorization to the security intro document

from #couchdb:

    dweremeichik
    Hi, I am curious as to what some of you do in reguards to securing data so that one organization/user cannot access another's data.

The docs for /{db}/_security cover this, but not in a verbose way.  I've added a new section to /intro/security.rst that covers the how and why of per-database authorization.