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
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8f22fe2025bf97bc2bd6e0a7b3acf97ab970a4e2

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