doc: Add descriptions for ports are occupied

If there are multiple runfvp instances running, default ports
(5001~5003) will automatically increase by 4.Add descriptions for this
case in User Manual.
And fix typo as well.

Issue-Id: SCM-4502
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I1e63ccfe2cc0ee065891b9ed86508d2a56114faa