doc: Add "nearest power of two" to PG rule-of-thumb

Following an IRC discussion, it emerged that it would be helpful
to explain the merit of choosing a number of PGs per pool that is
a power of two, to keep PGs at roughly equal sizes in case of
PG splits.

See http://irclogs.ceph.widodh.nl/index.php?date=2014-03-12 for the
original discussion.

Signed-off-by: Florian Haas <florian@hastexo.com>