doc: Update docstrings for freqz, freqz_plot. * freqz.m: List alternative calling forms. Eliminate extra space caused by expansion in ifnottex macro. Remove mention of plotting stop band. Add seealso link to freqz_plot. * freqz_plot.m: Use freq_norm as the name for the third input to match the code. Remove mention of plotting stop band. Add seealso link to freqz.