pgsql: Doc: improve documentation about exponentiation operator.

Doc: improve documentation about exponentiation operator.

Now that we're not having to wedge this into the straitjacket of
the old operator table format, we can add another example to
clarify the point about left-to-right associativity.

Per suggestion from mdione at grulic.org.ar.

https://postgr.es/m/162661954599.693.13700316547731859171@wrigleys.postgresql.org

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/227c4e57d6a70961e015ed4185facfc638afd048

Modified Files
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doc/src/sgml/func.sgml | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)