Say what the unilateral value saves over a set of deleted rows

[drametwally]

The deletion section explained why a tombstone lives in the unilateral value rather than in a
set of deleted row numbers, but not what that is worth. It is worth different amounts in
different structures, and one of them gains nothing from the usual argument.

• The matrix index gains the most. It derives a similarity for every row from that value, so
  excluding a deleted row incurs no cost beyond arithmetic the search already performs. A set
  would add a lookup for every row of every query.
• A scan index and the caches gain nearly as much. The tombstone is a field of the record they
  have already fetched, so there is no second structure to consult.
• An inverted index gains least. Its lists are ordered by unilateral value, so the tombstone
  cannot sit on the key the search prunes against. A deleted row is generated as a candidate
  and dropped only when verification reads its record. What it still gains is the absence of a
  second structure to maintain.

It also records a property that was load-bearing but unstated: a scorer that selects where it
scores and returns only the rows it kept has to apply deletion at that site, and the unilateral
values are already copied there for the arithmetic. A set would have to be copied too and kept
in step with every delete, or the selection would return deleted rows and leave the caller
short of what it asked for.

Prose only. No code changes.