pgsql: Add doc entry for the new GUC paramenter enable_group_by_reorder

Add doc entry for the new GUC paramenter enable_group_by_reordering

0452b461bc4 adds alternative orderings of group-by keys during the query
optimization. This new feature is controlled by the new GUC parameter
enable_group_by_reordering, which accidentally came without the documentation.
This commit adds the missing documentation for that GUC.

Reported-by: Bruce Momjian
Discussion: https://postgr.es/m/ZnDx2FYlba_OafQd%40momjian.us
Author: Andrei Lepikhov
Reviewed-by: Pavel Borisov, Alexander Korotkov

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/82e79ee46b1c880cb7376cf4399c9883c1ddfaea

Modified Files
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doc/src/sgml/config.sgml | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)