man: document cgcreate's -c and -S options

man: document cgcreate's -c and -S options

Add documentation about the -c (create scope) option and the -S
(set as default) option in the cgcreate man page. -c instructs
cgcreate to invoke cgroup_create_scope2() and create a systemd
scope.  -S (used in conjunction with -c) will set the provided
scope as the default for subsequent libcgroup operations.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Reviewed-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>