Update documentation of specgram

Mention that plotted data is in dB

Author: gitj

lib/matplotlib/axes.py

@@ -8573,11 +8573,12 @@ def specgram(self, x, NFFT=256, Fs=2, Fc=0, detrend=mlab.detrend_none,
                    cmap=None, xextent=None, pad_to=None, sides='default',
                    scale_by_freq=None, **kwargs)
 
-        Compute a spectrogram of data in *x*.  Data are split into
+        Compute and plot a spectrogram of data in *x*.  Data are split into
         *NFFT* length segments and the PSD of each section is
         computed.  The windowing function *window* is applied to each
         segment, and the amount of overlap of each segment is
-        specified with *noverlap*.
+        specified with *noverlap*. The spectrogram is plotted in decibels
+        as a colormap (using imshow).
 
         %(PSD)s