pgsql: doc: clarify handling of ts_headline() operators & extra words

doc: clarify handling of ts_headline() operators & extra words

Reported-by: Ngigi Waithaka & Alex Malek

Bug: 15172

Discussion: /messages/by-id/152461454026.19805.6310947081647212894@wrigleys.postgresql.org

Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/f5b75be030d588a119a19eca336527b7cfa77ec9

Modified Files
--------------
doc/src/sgml/textsearch.sgml | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)