doc/release-notes: v0.48.3argonaut

Signed-off-by: Sage Weil <sage@inktank.com>
osd: make missing head non-fatal during scrub

If we encounter a scrub without a preceeding head, warn instead of
crashing.  Note that this is still something we can't repair.

See #3705.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>