All methods must be documented
Round objects!
And so do I. Whilst I take your point that it's generally a good idea to document the methods that make up your public API, those methods will not be the same as the full set of methods to which an object responds.
Anyone who works with your code needs to see documentation covering how you expect the code to be used (though I'm as guilty as the next man of failing to do this on one of the major projects I'm working on, partly because the API is still in flux). Anyone who works on your code should be using the source like God intended. And if you're not competent to write clear code then what on earth makes you think you're competent to write clear comments in a far more ambiguous language?
