doc : remove brick usage change

This patch takes care of changing admin guide (admin_managing_volumes.md) for
remove-brick usage change. 3.6 version onwards remove-brick requires an explicit
option (start/force etc).

Change-Id: If6c5b0bf2c30c2b9a8c7644d314e2593006818c2
BUG: 1086737
Signed-off-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-on: http://review.gluster.org/7474
Reviewed-by: Lalatendu Mohanty <lmohanty@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>