pgsql: doc: clarify when expression indexes evaluate their expressions

doc: clarify when expression indexes evaluate their expressions

Only non-HOT updates evaluate the index expression.

Reported-by: Chris Lowder

Discussion: /messages/by-id/163967385701.26064.15365003480975321072@wrigleys.postgresql.org

Backpatch-through: 10

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e2e1bbde46a3509c3b7e830196f4314242925247

Modified Files
--------------
doc/src/sgml/indices.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)