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_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8b9d39254816195abb55ab204e39f8fa476aad23

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