doc: fix description of backward_scan
The description of the index property backward_scan was incorrect and
misleading; rectify.
Backpatch to 9.6 where the amutils functionality was introduced.
Branch
------
REL_10_STABLE
Details
-------
https://git.postgresql.org/pg/commitdiff/ee68541ce60dd585cbc53142ab9cd14bf10fdf8f
Modified Files
--------------
doc/src/sgml/func.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)