[doc] Fix OpenConnect MTU option and add Force DPD description. JB#51498
OpenConnect plugin uses --base-mtu which is different from --mtu. Change
description to correspond the option. Change from "VPN.MTU" to
"OpenConnect.MTU" to be more accurate.
Add description for the new Force DPD (Dead Peer Detection) option. This
is set as "OpenConnect.ForceDPD" option enabling --force-dpd.