Update comment for ReplicationSlot.last_saved_restart_lsn

Update comment for ReplicationSlot.last_saved_restart_lsn

Document that restart_lsn can go backwards and explain why this could happen.

Discussion: http://postgr.es/m/1d12d2-67235980-35-19a406a0%4063439497
Discussion: http://postgr.es/m/CAPpHfdvuyMrUg0Vs5jPfwLOo1M9B-GP5j_My9URnBX0B%3DnrHKw%40mail.gmail.com
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Co-authored-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>