doc: pg_num should always be a power of two

Signed-off-by: Kai Wagner <kwagner@suse.com>

doc/rados/operations/placement-groups.rst

@@ -433,9 +433,11 @@ You should then check if the result makes sense with the way you
 designed your Ceph cluster to maximize `data durability`_,
 `object distribution`_ and minimize `resource usage`_.
 
-The result should be **rounded up to the nearest power of two.**
-Rounding up is optional, but recommended for CRUSH to more evenly balance
-the number of objects among placement groups.
+The result should always be **rounded up to the nearest power of two.**
+The way the CRUSH algorithm is designed, a power of two leads to a more 
+evenly balance number of objects among placement groups. You could set it
+to a different value but you should be aware that this could result in uneven 
+balanced OSDs.
 
 As an example, for a cluster with 200 OSDs and a pool size of 3
 replicas, you would estimate your number of PGs as follows::