DOC: Change `nb` and `nb` to conventional m and n

Also point out that zero-based indexing means that the last coefficient is m-1, not m.

or could be changed to TeX?

$$
H(\omega) = \frac
{b_0j\omega^{(m-1)} + b_1 j\omega^{(m-2)} + \cdots + b_{(m-1)}}
{a_0j\omega^{(n-1)} + a_1 j\omega^{(n-2)} + \cdots + a_{(n-1)}}
$$