pgsql: Remove "invalid concatenation of jsonb objects" error case.

Remove "invalid concatenation of jsonb objects" error case.

The jsonb || jsonb operator arbitrarily rejected certain combinations
of scalar and non-scalar inputs, while being willing to concatenate
other combinations. This was of course quite undocumented. Rather
than trying to document it, let's just remove the restriction,
creating a uniform rule that unless we are handling an object-to-object
concatenation, non-array inputs are converted to one-element arrays,
resulting in an array-to-array concatenation. (This does not change
the behavior for any case that didn't throw an error before.)

Per complaint from Joel Jacobson. Back-patch to all supported branches.

Discussion: https://postgr.es/m/163099.1608312033@sss.pgh.pa.us

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/542248f9ddfd49f268b2edc4b6beeff7cf0b6aba

Modified Files
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doc/src/sgml/func.sgml | 11 +++--
src/backend/utils/adt/jsonfuncs.c | 85 +++++++++++++++++--------------------
src/test/regress/expected/jsonb.out | 36 +++++++++++++++-
src/test/regress/sql/jsonb.sql | 5 +++
4 files changed, 86 insertions(+), 51 deletions(-)