Document <targets> in start_scan and add example.
Document compatibility changes

It is possible now to specify different ports for different targets,
and run them all from the same task.
Each target, which can be a multi-host target, may have its own port list.
For this reason, get_scan_ports() receive the optional parameter 'target',
to return the corresponding port list for this target. In other case it
returns the port list of the first target for all targets.

For backward compatibility purposes it is still allow to pass a
target list and a port list as attributes, in which case the <targets>
element will be ignored. As before, exec_scan is called for each host
and the port list is the same for each host.