DOC: Describe return_counts keyword in np.unique docstring

Author: jaimefrio

numpy/lib/arraysetops.py

@@ -97,10 +97,11 @@ def unique(ar, return_index=False, return_inverse=False, return_counts=False):
     """
     Find the unique elements of an array.
 
-    Returns the sorted unique elements of an array. There are two optional
+    Returns the sorted unique elements of an array. There are three optional
     outputs in addition to the unique elements: the indices of the input array
-    that give the unique values, and the indices of the unique array that
-    reconstruct the input array.
+    that give the unique values, the indices of the unique array that
+    reconstruct the input array, and the number of times each unique value
+    comes up in the input array.
 
     Parameters
     ----------