DOC: Update admin generic driver network-options immutable
DOC: Update admin generic driver network-options immutable

This patch adds the behavior described in the bug #1713060
as known restriction in the generic driver.

Change-Id: Ic70fcd9f6cf159a960e0d75b4081c88af3d8973e
Closes-Bug: #1713060

openstack/manila,openstack/manila