Add usage for Power algorithm.

Refs #9574

Code/Mantid/docs/source/algorithms/Power-v1.rst

@@ -14,11 +14,37 @@ Exponent. When acting on an event workspace, the output will be a
 Workspace2D, with the default binning from the original workspace.
 
 Errors
-------
+######
 
 Defining the power algorithm as: :math:`y = \left ( a^b \right )`, we
 can describe the error as: :math:`s_{y} = by\left ( s_{a}/a \right )`,
 where :math:`s_{y}` is the error in the result *y* and :math:`s_{a}` is
 the error in the input *a*.
 
+
+Usage
+-----
+
+**Example - Square each Y value:**
+
+.. testcode::
+
+   # Create a 2 spectrum workspace with Y values 1->8
+   dataX = [0,1,2,3,4,
+            0,1,2,3,4]
+   dataY = [1,2,3,4,
+            5,6,7,8]
+   data_ws = CreateWorkspace(dataX, dataY, NSpec=2)
+   result_ws = Power(data_ws, 2)
+
+   print "Squared values of first spectrum:", result_ws.readY(0)
+   print "Squared values of second spectrum:", result_ws.readY(1)
+
+Output:
+
+.. testoutput::
+
+   Squared values of first spectrum: [  1.   4.   9.  16.]
+   Squared values of second spectrum: [ 25.  36.  49.  64.]
+
 .. categories::