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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master

Details
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https://git.postgresql.org/pg/commitdiff/7fa1e1ef741964eeb50f33d7c72622658bb7e5f4

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