BUG #11660: TSVector not returning partial match on word "out"

The following bug has been logged on the website:

Bug reference:      11660
Logged by:          David Levy
Email address:      david.g.levy@gmail.com
PostgreSQL version: 9.3.5
Operating system:   Windows 8 and RH Linux
Description:

I'm guessing that "out" is some kind of internal reserved word - I can't
find any documentation on this so submitting a bug until it can be
clarified/specified behavior:

-- First example returns true.
select to_tsvector('monkey outbreak') @@ to_tsquery('mon:*');

-- Second example returns false (bug?)
select to_tsvector('monkey outbreak') @@ to_tsquery('out:*');