pgsql: Doc: discourage use of partial indexes for poor-man's-partitioni

Doc: discourage use of partial indexes for poor-man's-partitioning.

Creating a bunch of non-overlapping partial indexes is generally
a bad idea, so add an example saying not to do that.

Back-patch to v10. Before that, the alternative of using (real)
partitioning wasn't available, so that the tradeoff isn't quite
so clear cut.

Discussion: /messages/by-id/CAKVFrvFY-f7kgwMRMiPLbPYMmgjc8Y2jjUGK_Y0HVcYAmU6ymg@mail.gmail.com

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bbce2912fa899de30783a6a76a6443b5a2aede8d

Modified Files
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doc/src/sgml/indices.sgml | 48 +++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 46 insertions(+), 2 deletions(-)