Update interfaces.md (#5279)

Add accessibility modifier info

Author: cartermp

docs/fsharp/language-reference/interfaces.md

@@ -12,7 +12,7 @@ ms.date: 05/16/2016
 ```fsharp
 // Interface declaration:
 [ attributes ]
-type interface-name =
+type [accessibility-modifier] interface-name =
     [ interface ]     [ inherit base-interface-name ...]
     abstract member1 : [ argument-types1 -> ] return-type1
     abstract member2 : [ argument-types2 -> ] return-type2
@@ -38,6 +38,8 @@ let class-name (argument-list) =
 ## Remarks
 Interface declarations resemble class declarations except that no members are implemented. Instead, all the members are abstract, as indicated by the keyword `abstract`. You do not provide a method body for abstract methods. However, you can provide a default implementation by also including a separate definition of the member as a method together with the `default` keyword. Doing so is equivalent to creating a virtual method in a base class in other .NET languages. Such a virtual method can be overridden in classes that implement the interface.
 
+The default accessibility for interfaces is `public`.
+
 You can optionally give each method parameter a name using normal F# syntax:
 
 [!code-fsharp[Main](../../../samples/snippets/fsharp/lang-ref-1/snippet24032.fs)]