documentation: revise placement group number guide

When a cluster has few OSDs (less than 50) propose a preselection of
values: as long as the number of placement groups is not too small nor
too large, it won't make much of a difference anyway.

Users of small clusters tend to blindly apply the (OSD*100)/(pool size)
formula and worry about chosing a wrong value because they do not
understand the tradeoffs. The preselection will hopefully save them from
this uncertainty.

Add an explanation of how placement groups relate to OSDs, CRUSH and
pools to help understand the tradeoffs. Explain the
tradeoffs (durability, distribution and resource usages) with examples.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>