pgsql: Clarify docs about limitations of constraint exclusion withpart

Clarify docs about limitations of constraint exclusion with partitions

The current wording can confuse the reader about constraint exclusion
being available at query execution, but this only applies to partition
pruning.

Reported-by: Shouyu Luo
Author: David Rowley
Reviewed-by: Chapman Flack, Amit Langote
Discussion: https://postgr.es/m/15629-2ef8b22e61f8333f@postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6af8c79868a3d51d9d0ae96c71596bba33780c59

Modified Files
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doc/src/sgml/ddl.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)