add doc

matlab/tfgm/utils/plot_mask.m

 function plot_mask(mask, hop, nbins, fs)
 
 %% plot_mask(mask, hop, nbins, fs)
-% % This function displays the mask
+% Plot time-frequency mask
 %
 % Inputs:
-%    - mask: binary mask
-%    - hop : length of time shift (int)
-%    - nbins: numbers of channels (int)
-%    - fs : sampling frequency
+%    - mask: Time-frequency mask
+%    - hop (int): Hop size
+%    - nbins (int):  Number of frequency bins
+%    - fs (int): Sampling frequency
 
 % Author : A. Marina KREME
 %%