doc|filter: fleshing out iir design section somewhat; still needs smoothing out
doc|filter: fleshing out iir design section somewhat; still needs smoothing out

biotrump/liquid-dsp,cjcliffe/liquid-dsp,andrepuschmann/liquid-dsp,andrepuschmann/liquid-dsp,wangning223/liquid-dsp,manuts/liquid-dsp,biotrump/liquid-dsp,biotrump/liquid-dsp,jgaeddert/liquid-dsp,cjcliffe/liquid-dsp,jgaeddert/liquid-dsp,wangning223/liquid-dsp,biotrump/liquid-dsp,JayKickliter/liquid-dsp,wangning223/liquid-dsp,manuts/liquid-dsp,manuts/liquid-dsp,jgaeddert/liquid-dsp,cjcliffe/liquid-dsp,andrepuschmann/liquid-dsp,andrepuschmann/liquid-dsp,JayKickliter/liquid-dsp,JayKickliter/liquid-dsp,wangning223/liquid-dsp,andrepuschmann/liquid-dsp,wangning223/liquid-dsp,jgaeddert/liquid-dsp,jgaeddert/liquid-dsp,manuts/liquid-dsp,cjcliffe/liquid-dsp,biotrump/liquid-dsp,cjcliffe/liquid-dsp,JayKickliter/liquid-dsp,manuts/liquid-dsp,JayKickliter/liquid-dsp