Add example for unreachable code

Fixes dotnet#937

I admit this rule confused me, and I asked to add the example.

After reading the updated, I'm ambivalent about adding the example. I don't want to move it into the bullet list, as that breaks the flow even more.

Let's discuss if it's needed at the meeting.

Author: BillWagner

standard/variables.md

@@ -343,6 +343,19 @@ The definite-assignment state of *v* on the control flow transfer to a reachable
   - If *v* is a pattern variable declared in the *switch_label*: “definitely assigned”.
   - Otherwise, the state of *v* is the same as the stat of *v* after *expr*.
 
+> *Example*: The third rule eliminates the need for the compiler to warn if an unassigned variable is accessed in unreachable code. The state of *b* is "definitely assigned" in the unreachable switch label `case 2 when b`.
+>
+> ```csharp
+> bool b;
+> switch (1) 
+> {
+>     case 2 when b: // b is definitely assigned here.
+>     break;
+> }
+> ```
+>
+> *end example*
+
 A consequence of these rules is that a pattern variable declared in a *switch_label* will be “not definitely assigned” in the statements of its switch section if it is not the only reachable switch label in its section.
 
 > *Example*: