pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1cc34640cabcb32b4f062673cce1d6b1819d492d

Modified Files
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doc/src/sgml/func.sgml | 46 +++++++++++++++++-----------------------------
1 file changed, 17 insertions(+), 29 deletions(-)