docs: mention migration issue of which credentials are used

docs: mention migration issue of which credentials are used

Based on a report by Seth Vidal.  Just because _you_ can use virsh
to connect to both source and destinations does not mean that libvirtd
on the source (aka _root_) can likewise connect to the destination;
this matters when setting up a peer-to-peer migration instead of a
native one.

* docs/migration.html.in: Mention that in peer-to-peer, the owner
of the source libvirtd (usually root) must be able to connect to
the destination.