migration/multifd: Document the reason to sync for save_setup()

It's not straightforward to see why src QEMU needs to sync multifd during
setup() phase.  After all, there's no page queued at that point.

For old QEMUs, there's a solid reason: EOS requires it to work.  While it's
clueless on the new QEMUs which do not take EOS message as sync requests.

One will figure that out only when this is conditionally removed.  In fact,
the author did try it out.  Logically we could still avoid doing this on
new machine types, however that needs a separate compat field and that can
be an overkill in some trivial overhead in setup() phase.

Let's instead document it completely, to avoid someone else tries this
again and do the debug one more time, or anyone confused on why this ever
existed.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241206224755.1108686-8-peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

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