Doc: improve description of geometric multiplication/division.

Doc: improve description of geometric multiplication/division.

David Johnston reminded me that the per-point calculations being done
by these operators are equivalent to complex multiplication/division.
(Once I would've recognized that immediately, but it's been too long
since I did any of that sort of math.)

Also put in a footnote mentioning that "rotation" of a box doesn't do
what you might expect, as I'd griped about in the referenced thread.

Discussion: https://postgr.es/m/158110996889.1089.4224139874633222837@wrigleys.postgresql.org