pgsql: doc: Mention all SELECT privileges required by INSERT ... ON CON

doc: Mention all SELECT privileges required by INSERT ... ON CONFLICT.

On the INSERT page, mention that SELECT privileges are also required
for any columns mentioned in the arbiter clause, including those
referred to by the constraint, and clarify that this applies to all
forms of ON CONFLICT, not just ON CONFLICT DO UPDATE.

Author: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Reviewed-by: Viktor Holmberg <v(at)viktorh(dot)net>
Discussion: https://postgr.es/m/CAEZATCXGwMQ+x00YY9XYG46T0kCajH=21QaYL9Xatz0dLKii+g@mail.gmail.com
Backpatch-through: 14

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7233e70537da6a9e6f1f383c3a2bed73fc121b62

Modified Files
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doc/src/sgml/ref/insert.sgml | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)