pgsql: Doc: improve examples for json_populate_record() and related fun

Doc: improve examples for json_populate_record() and related functions.

Make these examples self-contained by providing declarations of the
user-defined row types they rely on.  There wasn't room to do this
in the old doc format, but now there is, and I think it makes the
examples a good bit less confusing.

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/124772e3cac6d3bf6c1f6d32518209c6a3fdd3eb

Modified Files
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doc/src/sgml/func.sgml | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)