pgsql: Add comments and a missing CHECK_FOR_INTERRUPTS in ts_headline.

Add comments and a missing CHECK_FOR_INTERRUPTS in ts_headline.

I just spent an annoying amount of time reverse-engineering the
100%-undocumented API between ts_headline and the text search
parser's prsheadline function.  Add some commentary about that
while it's fresh in mind.  Also remove some unused macros in
wparser_def.c.

While at it, I noticed that when commit 78e73e875 added a
CHECK_FOR_INTERRUPTS call in TS_execute_recurse, it missed
doing so in the parallel function TS_phrase_execute, which
surely needs one just as much.

Back-patch because of the missing CHECK_FOR_INTERRUPTS.
Might as well back-patch the rest of this too.

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0353db996e37c6b923863ee648f4d2f153bf619d

Modified Files
--------------
src/backend/tsearch/ts_parse.c      | 13 ++++++++
src/backend/tsearch/wparser_def.c   |  4 ---
src/backend/utils/adt/tsvector_op.c |  3 ++
src/include/tsearch/ts_public.h     | 61 ++++++++++++++++++++++++++-----------
4 files changed, 60 insertions(+), 21 deletions(-)