Please explain the --volume parameter in config kubelet service to enable FlexVolume driver

**Is your feature request related to a problem?/Why is this needed**
I don't understand the purpose of this line and where to put it.
--volume=/etc/kubernetes/volumeplugins:/etc/kubernetes/volumeplugins:rw \
There is no such file
/etc/systemd/system/kubelet.service.
There are /var/lib/kubelet/config.yaml and /etc/systemd/system/kubelet.service.d/10-kubeadm.conf and /etc/default/kubelet containing KUBELET_EXTRA_ARGS=...

**Describe the solution you'd like in detail**
clarify the documentation to explain this flag and its purpose. Clarify where to add this flag in recent installations of kubelet that are driven by config files.

**Describe alternatives you've considered**
adding this argument to KUBELET_EXTRA_ARGS --volume=/etc/kubernetes/volumeplugins:/etc/kubernetes/volumeplugins:rw

**Additional context**

Start by reviewing the existing documentation for enabling the FlexVolume driver and compare it with the configuration paths named in the issue: /var/lib/kubelet/config.yaml, /etc/systemd/system/kubelet.service.d/10-kubeadm.conf, and /etc/default/kubelet. Done means the --volume purpose and the correct placement for recent kubelet installations are clearly explained.

Written by the indexing model from the issue text.