pgsql: Update obsolete comments in ResultRelInfo struct.

Update obsolete comments in ResultRelInfo struct.

Commit c5b7ba4e6 changed things so that the ri_RootResultRelInfo field
of this struct is set for both partitions and inheritance children and
used for tuple routing and transition capture (before that commit, it
was only set for partitions to route tuples into), but failed to update
these comments.

Author: Etsuro Fujita <etsuro(dot)fujita(at)gmail(dot)com>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAPmGK14NF5CcdCmTZpxrvpvBiT0y4EqKikW1r_wAu1CEHeOmUA%40mail.gmail.com
Backpatch-through: 14

Branch
------
REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/eb13f3d2e05cc03e41058c1cfbe9749a79612cc9

Modified Files
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src/include/nodes/execnodes.h | 12 +++++-------
1 file changed, 5 insertions(+), 7 deletions(-)