migration: Add documentation for fdset with multifd + file

With the last few changes to the fdset infrastructure, we now allow
multifd to use an fdset when migrating to a file. This is useful for
the scenario where the management layer wants to have control over the
migration file.

By receiving the file descriptors directly, QEMU can delegate some
high level operating system operations to the management layer (such
as mandatory access control). The management layer might also want to
add its own headers before the migration stream.

Document the "file:/dev/fdset/#" syntax for the multifd migration with
mapped-ram. The requirements for the fdset mechanism are:

- the fdset must contain two fds that are not duplicates between
  themselves;

- if direct-io is to be used, exactly one of the fds must have the
  O_DIRECT flag set;

- the file must be opened with WRONLY on the migration source side;

- the file must be opened with RDONLY on the migration destination
  side.

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

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