doc: Clarify RLS policies applied for ON CONFLICT DO NOTHING.

doc: Clarify RLS policies applied for ON CONFLICT DO NOTHING.

On the CREATE POLICY page, the description of per-command policies
stated that SELECT policies are applied when an INSERT has an ON
CONFLICT DO NOTHING clause. However, that is only the case if it
includes an arbiter clause, so clarify that.

While at it, also clarify the comment in the regression tests that
cover this.

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