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
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/38d30a14b05e0cc2996fd311d94d7ae4fe2122aa

Modified Files
--------------
doc/src/sgml/func.sgml              | 22 +++++++++-
src/backend/utils/adt/jsonfuncs.c   | 83 +++++++++++++++++--------------------
src/test/regress/expected/jsonb.out | 36 +++++++++++++++-
src/test/regress/sql/jsonb.sql      |  5 +++
4 files changed, 99 insertions(+), 47 deletions(-)