Information about unwrapping single case unions (dotnet#2912)

* Unwrapping single case unions

I think this is important to know because this small details makes unions even more attractive to use.

Form and details of this can be discussed.

cc @cartermp

* Apply suggested changes.

Signed-off-by: realvictorprm <mueller.vpr@gmail.com>

* Apply last changes

* All your typos belong to me

* Fixing German habits

* Missed that typo!

Author: realvictorprm

Diff for: docs/fsharp/language-reference/discriminated-unions.md

@@ -81,6 +81,24 @@ let getShapeHeight shape =
 
 Normally, the case identifiers can be used without qualifying them with the name of the union. If you want the name to always be qualified with the name of the union, you can apply the [RequireQualifiedAccess](https://msdn.microsoft.com/library/8b9b6ade-0471-4413-ac5d-638cd0de5f15) attribute to the union type definition.
 
+### Unwrapping Discriminated Unions
+
+In F# Discriminated Unions are often used in domain-modeling for wrapping a single type. It's easy to extract the underlying value via pattern matching as well. You don't need to use a match expression for a single case:
+```fsharp
+let ([UnionCaseName] [values]) = [UnionValue]
+```
+
+The following example demonstrates this:
+
+```fsharp
+type ShaderProgram = | ShaderProgram of id:int
+
+let someMethodUsingShaderProgram shaderProgram =
+    let (ShaderProgram id) = shaderProgram
+    // Use the unwrapped value
+    ..
+```
+
 ## Struct Discriminated Unions
 
 Starting with F# 4.1, you can also represent Discriminated Unions as structs.  This is done with the `[<Struct>]` attribute.