rdmacg: fix kernel-doc warnings in rdmacg

Fix all kernel-doc warnings in rdmacg:

kernel/cgroup/rdma.c:209: warning: Function parameter or member 'cg'
not described in 'rdmacg_uncharge_hierarchy'
kernel/cgroup/rdma.c:230: warning: Function parameter or member 'cg'
not described in 'rdmacg_uncharge'

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>