ceph: fix the document of OSD management
ceph: fix the document of OSD management

Removing a disk is not mandatory after the data in the corresponding
OSD is moved to other OSDs. It's better to just let users to know it's
safe to remove the disk.

Signed-off-by: Satoru Takeuchi <0be322b419f8cd46ee75447832ef0476dff222bc@gmail.com>

rook/nfs,rook/rook,rook/rook,rook/rook,rook/rook,rook/rook,rook/nfs,rook/nfs