> In every case I can think of, I've wanted (0).numbits() to be 0.

Me too, in most cases, though I've encountered the occasional case where 
raising ValueError would be more appropriate and would catch some bugs 
that might otherwise pass silently.

So I agree that (0).numbits() should be 0, but I think there's enough 
potential ambiguity that there should be a sentence in the documentation  
making this explicit.  Two of the most obvious (wrong) formulas for 
numbits are: (1) numbits(n) = ceiling(log_2(n)), and (2) numbits(n) = 
len(bin(n))-2, but neither of these formulas gives the right result for 
0, or for negative numbers for that matter.

> The explanation in the docstring can probably be improved. What other 
documentation is needed (where)?

The docstring looked okay to me.  There should be more comprehensive 
ReST documentation in the Doc/ directory somewhere, probably in 
Doc/library/stdtypes.rst