Clarify impact on how GAE instance hours are spent

I recently started playing around with this, very neat piece of
functionality.

One thing I noticed is that whenever I use a DB connection in my GAE
instances the CPU hours seem to be consumed at a much higher rate.
This is an empirical observation of course.

I remember reading on the documentation that using GAE applications
with master/slave replication instead of HRD could hurt mysql
performance, which hints that the GAE instance could be the underlying
instance powering the DB. If this is the case, I think this should be
mentioned in the documentation as this has a potential impact on how
the instance hours are spent.