[ZODB-Dev] Updated BTree docs

[Tim Peters]

Because the secret requirements for what you can get away with in objects
used as BTree/Bucket keys (TreeSet/Set elements) keep getting violated for
some reason <wink>, Jeremy & I conspired to make some new docs appear:

    http://www.zope.org/Wikis/ZODB/guide/zodb.html

Section 5.3 (BTrees Package) has been updated, and section 5.3.1 (Total
Ordering and Persistence) is new.  This is definitely an area where an ounce
of prevention is better than three days of debugging pain, so give it a
read!