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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8344979d5bbc63aa8fb8c804bd933c887a2abb58

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