I found such query in some of the working repository:

SELECT id, full_name_tsvector <=> plainto_tsquery('English', 'test') AS rank
FROM names_tsvectors 
WHERE "full_name_tsvector" @@ plainto_tsquery('English', 'test')
ORDER BY rank LIMIT 1000

The query gives result:

id   |rank     |
-----+---------+
 6574|13.159472|
25653|13.159472|
 5730|13.159472|
21924|13.159472|
 6578|13.159472|
 5813|13.159472|
22188| 16.44934|
21959| 16.44934|
20360| 16.44934|

So my question is – what does operator <=> do actually? I haven’t found anything about that operator except that it does not exist in Postgres.