Обсуждение: pgsql: Doc: add a bit to indices.sgml about what is an indexable clause

Doc: add a bit to indices.sgml about what is an indexable clause.

We didn't explain this clearly until somewhere deep in the
"Extending SQL" chapter, but really it ought to be mentioned
in the introductory material too.

Discussion: https://postgr.es/m/4097442.1694967650@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b4c147ac6fd5238d2c49b6369d555f29dbfc9144

Modified Files
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doc/src/sgml/indices.sgml | 35 ++++++++++++++++++++++++++++++++++-
1 file changed, 34 insertions(+), 1 deletion(-)