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.

src/backend/utils/adt/tsvector_op.c

@@ -1617,6 +1617,9 @@ TS_phrase_execute(QueryItem *curitem, void *arg, uint32 flags,
 	/* since this function recurses, it could be driven to stack overflow */
 	check_stack_depth();
 
+	/* ... and let's check for query cancel while we're at it */
+	CHECK_FOR_INTERRUPTS();
+
 	if (curitem->type == QI_VAL)
 		return chkcond(arg, (QueryOperand *) curitem, data);