Update restrictions on parameters

Fixes #896.

Author: jskeet

standard/variables.md

@@ -1224,8 +1224,8 @@ A `new` expression that invokes a constructor obeys the same rules as a method i
 
 #### 9.7.2.9 Limitations on reference variables
 
-- Neither a `ref` parameter, nor a `ref` local, nor a parameter or local of a `ref struct` type shall be captured by lambda expression or local function.
-- Neither a `ref` parameter nor a parameter of a `ref struct` type shall be an argument for an iterator method or an `async` method.
+- Neither a reference parameter, nor an output parameter, nor an input parameter, nor a `ref` local, nor a parameter or local of a `ref struct` type shall be captured by lambda expression or local function.
+- Neither a reference parameter, nor an output parameter, nor an input parameter, nor a parameter of a `ref struct` type shall be an argument for an iterator method or an `async` method.
 - Neither a `ref` local, nor a local of a `ref struct` type shall be in context at the point of a `yield return` statement or an `await` expression.
 - For a ref reassignment `e1 = ref e2`, the ref-safe-context of `e2` must be at least as wide a context as the *ref-safe-context* of `e1`.
 - For a ref return statement `return ref e1`, the ref-safe-context of `e1` must be the caller-context.