docs: tweak 'virsh edit' wording

docs: tweak 'virsh edit' wording

I was wondering why 'virsh edit' didn't support the same
'--inactive' option as 'virsh dumpxml'; reading the source
code showed that --inactive was already implied, and that
the only way to alter a running guest rather than affecting
next boot is by hot-plugging individual devices, or by
something complex like saving the guest and modifying the
save image.

* tools/virsh.pod (define, edit): Mention behavior when guest is
already running.