pgsql: Document jsonpath .** accessor with nesting level filter

Document jsonpath .** accessor with nesting level filter

It appears that some variants of .** jsonpath accessor are undocumented. In
particular undocumented variants are:

.**{level}
.**{lower_level to upper_level}
.**{lower_level to last}

This commit adds missing documentation for them.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0e3dad770d3eefc4ee6cd562d9c25b61c263a6e

Modified Files
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doc/src/sgml/json.sgml | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)