[function] missing document function::assign()

[Akira Takahashi <faithandbrave@…>]

boost::function::assign() member function is undocument. This function can assign allocator object one of the few way. I hope documentation this function.

#include <cassert>
#include <memory>
#include <boost/function.hpp>

struct increment {
    int operator()(int x) const { return x + 1; }
};

int main()
{
    boost::function<int(int)> f;
    f.assign(increment(), std::allocator<increment>());

    assert(f(1) == 2);
}