[skip ci] Add comments for etcd (#8366)

Signed-off-by: shiyu22 <shiyu.chen@zilliz.com>

configs/milvus.yaml

@@ -9,7 +9,7 @@
 # is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 # or implied. See the License for the specific language governing permissions and limitations under the License.
 
-
+# Related configuration of etcd, used to store Milvus metadata.
 etcd:
   endpoints:
     - localhost:2379