doc: pg_num should always be a power of two
I updated the pg_num section in the docs just a little to be more strict.
I think we should make it crystal clear that pg num should be a power of two and only if there's a real good reason and someone knows exactly why they need a different value they can proceed.
I think this is the first step of several to make this clearer to the end user. Next step would be to show a Health WARN and also to flag the pool creation red in the dashboard if someone would like to add a pool and the pg count isn't a power of two.
Why do I would like to be more strict about it? We've seen more and more clusters were the osd variation was really high. The fix to that was always to set the pg_num + balancer. Also the pg autoscaler only uses power of two so we shouldn't even encourage people to use something different right from the beginning or to phrase i differently, we should protect the end-user right away.