pgsql: Fix two errors in extraction of GIN indexquals from jsonpath que

Fix two errors in extraction of GIN indexquals from jsonpath queries.

extract_jsp_bool_expr applied De Morgan's laws incorrectly when
considering an AND operator underneath a NOT. If one AND input
was implementable as a GIN indexqual while the other was not,
we derived an incomplete indexqual, leading to some rows not
being returned that should be.

extract_jsp_bool_expr also failed to check for methods being
applied to literals, so that for example '$.x == "42".double()'
was treated as '$.x == "42"' not '$.x == 42'. The indexqual
thus selected the wrong rows, all of which would fail qual
recheck, leading to no rows being returned.

While we're here, improve the comments a trifle.

Reported-by: Chinmay Kanchi <cgkanchi(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAJqPDh9Cm=x8eYZ1Jf=_avd2WEqc=0t_f7_SO4igo-CKPi25_Q@mail.gmail.com
Discussion: https://postgr.es/m/CAJqPDh-tu5KY66KU8cvAEg3DQrZLzDZwjkLLa42zD+_Xm1WJAg@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d987ad61d3b1cc3881d85352761607f496e45751

Modified Files
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src/backend/utils/adt/jsonb_gin.c | 28 ++++++++++++++++++++++------
src/test/regress/expected/jsonb.out | 30 ++++++++++++++++++++++++++++++
src/test/regress/sql/jsonb.sql | 5 +++++
3 files changed, 57 insertions(+), 6 deletions(-)