doc: Took out "future" reference to namespaces.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: PG splitting added to docs.

fixes: #6727

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added commentary to configure pg defaults. Clarified size commentary.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge branch 'master' of https://github.com/ceph/ceph