Clarify that only instance fields are relevant for unmanaged types (#914)

Fixes #891

Author: jskeet

standard/types.md

@@ -696,8 +696,8 @@ unmanaged_type
     ;
 ```
 
-An *unmanaged_type* is any type that isn’t a *reference_type*, a *type_parameter*, or a constructed type, and contains no fields whose type is not an *unmanaged_type*. In other words, an *unmanaged_type* is one of the following:
+An *unmanaged_type* is any type that isn’t a *reference_type*, a *type_parameter*, or a constructed type, and contains no instance fields whose type is not an *unmanaged_type*. In other words, an *unmanaged_type* is one of the following:
 
 - `sbyte`, `byte`, `short`, `ushort`, `int`, `uint`, `long`, `ulong`, `char`, `float`, `double`, `decimal`, or `bool`.
 - Any *enum_type*.
-- Any user-defined *struct_type* that is not a constructed type and contains fields of *unmanaged_type*s only.
+- Any user-defined *struct_type* that is not a constructed type and contains instance fields of *unmanaged_type*s only.