Added documentation for parameters vmin and vmax inside specgram function. (matplotlib#28613)

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: Alsosa

lib/matplotlib/axes/_axes.py

@@ -8136,6 +8136,11 @@ def specgram(self, x, NFFT=None, Fs=None, Fc=None, detrend=None,
         data : indexable object, optional
             DATA_PARAMETER_PLACEHOLDER
 
+        vmin, vmax : float, optional
+            vmin and vmax define the data range that the colormap covers.
+            By default, the colormap covers the complete value range of the
+            data.
+
         **kwargs
             Additional keyword arguments are passed on to `~.axes.Axes.imshow`
             which makes the specgram image. The origin keyword argument