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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7a4bdd986ab9a10f3bc2d8875be6e154b63a1918

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