Clarify Optional Parameters and Nullable Optional Parameter Behavior in C#.

docs/csharp/programming-guide/classes-and-structs/named-and-optional-arguments.md

@@ -70,13 +70,14 @@ The following code implements the examples from this section along with some add
 
 ## Optional arguments
 
-The definition of a method, constructor, indexer, or delegate can specify its parameters are required or optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters.
+The definition of a method, constructor, indexer, or delegate can specify its parameters are required or optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters. Additionally, nullable reference types (`T?`) implicitly have `null` as their default value.
 
 Each optional parameter has a default value as part of its definition. If no argument is sent for that parameter, the default value is used. A default value must be one of the following types of expressions:
 
 - a constant expression;  
 - an expression of the form `new ValType()`, where `ValType` is a value type, such as an [enum](../../language-reference/builtin-types/enum.md) or a [struct](../../language-reference/builtin-types/struct.md);
 - an expression of the form [default(ValType)](../../language-reference/operators/default.md),  where `ValType` is a value type.
+- For nullable value types or nullable reference types (`T?`), the default value is always `null`.
 
 Optional parameters are defined at the end of the parameter list, after any required parameters. If the caller provides an argument for any one of a succession of optional parameters, it must provide arguments for all preceding optional parameters. Comma-separated gaps in the argument list aren't supported. For example, in the following code, instance method `ExampleMethod` is defined with one required and two optional parameters.
 
@@ -85,7 +86,7 @@ Optional parameters are defined at the end of the parameter list, after any requ
 The following call to `ExampleMethod` causes a compiler error, because an argument is provided for the third parameter but not for the second.
 
 ```csharp
-//anExample.ExampleMethod(3, ,4);
+// anExample.ExampleMethod(3, ,4);
 ```
 
 However, if you know the name of the third parameter, you can use a named argument to accomplish the task.
@@ -99,7 +100,7 @@ IntelliSense uses brackets to indicate optional parameters, as shown in the foll
 ![Screenshot showing IntelliSense quick info for the ExampleMethod method.](./media/named-and-optional-arguments/optional-examplemethod-parameters.png)
 
 > [!NOTE]
-> You can also declare optional parameters by using the .NET <xref:System.Runtime.InteropServices.OptionalAttribute> class. `OptionalAttribute` parameters do not require a default value. However, if a default value is desired, take a look at <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> class.
+> You can also declare optional parameters by using the .NET <xref:System.Runtime.InteropServices.OptionalAttribute> class. `OptionalAttribute` parameters do not require a default value. However, if a default value is desired, take a look at <xref:System.Runtime.InteropServices.DefaultParameterValueAttribute> class. Additionally, nullable types (`T?`) implicitly default to `null` without needing to use these attributes.
 
 ### Example