pgsql: Be clearer about when jsonapi's need_escapes is needed

Be clearer about when jsonapi's need_escapes is needed

Most operations beyond pure json parsing need to set need_escapes to
true to get access to field names and string scalars. Document this
fact more explicitly.

Slightly tweaked patch from:

Author: Corey Huinker <corey.huinker@gmail.com>

Discussion: /messages/by-id/CADkLM=c49Vkfg2+A8ubSuEtaGEjuaKZXCA6SrXA8kdwHjx3uxQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ea5ff5833c7d4ae727a5acfc590c848b520775d0

Modified Files
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src/common/jsonapi.c | 6 +++++-
src/include/common/jsonapi.h | 15 +++++++++++++--
2 files changed, 18 insertions(+), 3 deletions(-)