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
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e1dd1f924e1990c604ce353c23fc6c0d2dc996a8

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