pgsql: Fix conversion of JSON strings to JSON output columns injson_to

Fix conversion of JSON strings to JSON output columns in json_to_record().

json_to_record(), when an output column is declared as type json or jsonb,
should emit the corresponding field of the input JSON object.  But it got
this slightly wrong when the field is just a string literal: it failed to
escape the contents of the string.  That typically resulted in syntax
errors if the string contained any double quotes or backslashes.

jsonb_to_record() handles such cases correctly, but I added corresponding
test cases for it too, to prevent future backsliding.

Improve the documentation, as it provided only a very hand-wavy
description of the conversion rules used by these functions.

Per bug report from Robert Vollmert.  Back-patch to v10 where the
error was introduced (by commit cf35346e8).

Note that PG 9.4 - 9.6 also get this case wrong, but differently so:
they feed the de-escaped contents of the string literal to json[b]_in.
That behavior is less obviously wrong, so possibly it's being depended on
in the field, so I won't risk trying to make the older branches behave
like the newer ones.

Discussion: https://postgr.es/m/D6921B37-BD8E-4664-8D5F-DB3525765DCD@vllmrt.net

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b6f5689aadf42b9afa3bf972d3deee839541a3ba

Modified Files
--------------
doc/src/sgml/func.sgml              | 87 +++++++++++++++++++++++++++----------
src/backend/utils/adt/jsonfuncs.c   | 37 +++++++---------
src/test/regress/expected/json.out  | 36 +++++++++++++++
src/test/regress/expected/jsonb.out | 36 +++++++++++++++
src/test/regress/sql/json.sql       |  7 +++
src/test/regress/sql/jsonb.sql      |  7 +++
6 files changed, 167 insertions(+), 43 deletions(-)