Confirmed.  More interestingly, nowadays (at least in 3.5) test_pdb.py depends on this bug.  If we really clear f->f_trace when the trace function returns None, then test_pdb_until_command_for_generator() fails.  This is because pdb.py incorrectly thinks there is no breakpoint in the generator function, and returns None.  This doesn't actually clear anything, and so it works anyway.

I'd suggest to fix the documentation to reflect the actual behavior of all versions from 2.3 to (at least) 3.5.