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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/499342e81b0fba471889439e47e0c37fe9e62bc0

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