docs: Update for 11.18 FVP release

 * Add note to release 4.0 changelog
 * Update link to user mode networking page in FVP docs

Issue-Id: SCM-4388
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I64a119a95a3aeec679faac84d47ea232276b8c83