Clarify cross correlation documentation

Author: dstansby

lib/matplotlib/axes/_axes.py

@@ -1673,8 +1673,7 @@ def acorr(self, x, **kwargs):
             normalization.
 
         normed : boolean, optional, default: True
-            if True, normalize the data by the autocorrelation at the 0-th
-            lag.
+            if True, input vectors are normalised to unit length.
 
         usevlines : boolean, optional, default: True
             if True, Axes.vlines is used to plot the vertical lines from the
@@ -1723,6 +1722,8 @@ def xcorr(self, x, y, normed=True, detrend=mlab.detrend_none,
         """
         Plot the cross correlation between *x* and *y*.
 
+        The correlation with lag k is defined as sum_n x[n+k] * conj(y[n]).
+
         Parameters
         ----------
 
@@ -1737,8 +1738,7 @@ def xcorr(self, x, y, normed=True, detrend=mlab.detrend_none,
             normalization.
 
         normed : boolean, optional, default: True
-            if True, normalize the data by the autocorrelation at the 0-th
-            lag.
+            if True, input vectors are normalised to unit length.
 
         usevlines : boolean, optional, default: True
             if True, Axes.vlines is used to plot the vertical lines from the