FWIW, grepping through Lib/test reveals the following statistics:

assertFalse: 83
assertTrue: 97
failUnless: 684
assert_: 1977

Not having assertTrue/assertFalse methods in the Library Reference does 
not discourage people from using them given that they show up without 
any warning in help().  (Moreover, assertTrue shows up before assert_.)

I would say it should be documented.  After all it would only take a two 
line patch (see attached). 

(I agree with OP that assertTrue is clearer than assert_ . Although I 
know that '_' is only there to avoid a conflict with the assert keyword,   
assert_ always stand out as different from the other TestCase methods.)