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_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/da3b61368177a5a9ced5412592932b8e8429b302

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