doc: Deleted redundant text string.

Signed-off-by: John Wilkins <jowilki@redhat.com>
doc: Added clarifications and added additional preflight steps for CentOS/RHEL.

Fixes: #8995

Signed-off-by: John Wilkins <jowilki@redhat.com>
doc: Added feedback.

Signed-off-by: John Wilkins <jowilki@redhat.com>
Merge pull request #2473 from ceph/wip-doc-preflight

doc: Revised the Preflight Doc

Reviewed-by: John Wilkins <jowilkin@redhat.com>