not officially documented use of setweight??

I think I've learned how to use pg text search from Oleg and Teodor
documentation since I've found on my code this use of setweight:

query := query &&
  setweight(configuration, 'banana apple orange', 'B', '&');

But I can't find any trace of this use in official postgres docs.
The docs just says:

setweight(vector tsvector, weight "char") returns tsvector

Am I missing them? Is that use "supported" in future versions?

--
Ivan Sergio Borgonovo
http://www.webthatworks.it