Add Throws for exceptions to the docs (#795)

* Add Throws for exceptions to the docs
* Update ExtractDocs.fs to import ExceptionExtensions

build/ExtractDocs.fs

@@ -37,6 +37,7 @@ using System.Collections.Generic;
 using System.ComponentModel;
 using NSubstitute.Extensions;
 using NSubstitute.Compatibility;
+using NSubstitute.ExceptionExtensions;
 
 namespace NSubstitute.Samples {
     public class Tests_%s {

docs/help/_posts/2010-05-02-throwing-exceptions.markdown

@@ -3,8 +3,6 @@ title: Throwing exceptions
 layout: post
 ---
 
-[Callbacks](/help/callbacks) can be used to throw exceptions when a member is called.
-
 <!--
 ```requiredcode
 public interface ICalculator { int Add(int a, int b); }
@@ -13,6 +11,25 @@ ICalculator calculator;
 ```
 -->
 
+The `Throws` and `ThrowsAsync` helpers in the `NSubstitute.ExceptionExtensions` namespace can be used to throw exceptions when a member is called.
+
+```csharp
+//For non-voids:
+calculator.Add(-1, -1).Throws(new Exception()); // Or .Throws<Exception>()
+
+//For voids and non-voids:
+calculator
+    .When(x => x.Add(-2, -2))
+    .Throw(x => new Exception()); // Or .Throw<Exception>() -  - don't use .Throw*s* in this case
+
+//Both calls will now throw.
+Assert.Throws<Exception>(() => calculator.Add(-1, -1));
+Assert.Throws<Exception>(() => calculator.Add(-2, -2));
+```
+
+### Returns
+Another way is to use the underlying method, `.Returns`. See also [Callbacks](/help/callbacks).
+
 ```csharp
 //For non-voids:
 calculator.Add(-1, -1).Returns(x => { throw new Exception(); });
@@ -25,4 +42,4 @@ calculator
 //Both calls will now throw.
 Assert.Throws<Exception>(() => calculator.Add(-1, -1));
 Assert.Throws<Exception>(() => calculator.Add(-2, -2));
-```
+```