Better explain why COMM_MEMBS might be incorporated into SIGHTINGS

Better explain why COMM_MEMBS might be incorporated into SIGHTINGS

It would be even better to rename SIGHTINGS to MEMBERS, and have it
show community membership.  The build_sightings() function would be
renamed build_members().  And SIGHTINGS would then be a view of
MEMBERS, excluding the rows that exist in MEMBERS only because of
COMM_MEMBS.

But this probably also requires that build_members() always look at
COMM_MEMBS, last.  And have some sort of special Step value.
Otherwise we wouldn't know what MEMBERS.Step value to exclude when
constructing the SIGHTINGS view.

All this is, at present, not the best use of our available time.