pgsql: Doc: cover index CONCURRENTLY causing errors in INSERT ... ON CO

Doc: cover index CONCURRENTLY causing errors in INSERT ... ON CONFLICT.

Author: Mikhail Nikalayeu <mihailnikalayeu@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/CANtu0ojXmqjmEzp-=aJSxjsdE76iAsRgHBoK0QtYHimb_mEfsg@mail.gmail.com
Backpatch-through: 13

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4c2895629f6fface95085121809ce34c5e6d6503

Modified Files
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doc/src/sgml/ref/insert.sgml         | 9 +++++++++
src/backend/optimizer/util/plancat.c | 5 +++++
2 files changed, 14 insertions(+)