Low: exportfs: increase documented stop time.

This increases the recommended stop time from 10s
to 120s. This will help mitigate users running into
the problem where they need to wait out the nfsv4 lease
time during stop, but the stop timeout is too short which
causes the resource to fail. It is very common for the nfsv4
lease timeout to be 90 seconds. Increasing the stop timeout
to 120s gives us plenty of room to work with here for the
majority of use-cases.