domain: Document VIR_DOMAIN_XML_MIGRATABLE
    
    Commit 28f8dfdc (1.0.0) added a flag to virDomainGetXMLDesc, but
    failed to document its effects.  And considering that the
    MIGRATABLE flag has been the source of past bugs (CVE-2014-7823,
    fixed in commit b1674ad5 (1.2.11), or even cf2d4c60 (1.2.13) where
    flag mismatch broke virsh edit), make the wording wishy-washy
    enough to discourage using the flag casually, by mentioning that
    the resulting XML is more for internal use than for validation
    against the schema.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>