migration: improve documentation of postcopy-ram

migration: improve documentation of postcopy-ram

This capability must have the same value on both source and destination,
otherwise migration fails (commit 875fcd013ab6 "migration: incoming
postcopy advise sanity checks").

Let's write it down in various places where postcopy-ram is documented.

Signed-off-by: Greg Kurz <[email protected]>
Message-Id: <151801810352.29167.4832480228518630626[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>