Update standard/types.md

Author: jskeet

standard/types.md

@@ -715,7 +715,7 @@ unmanaged_type
     ;
 ```
 
-An *unmanaged_type* is any type that isn’t a *reference_type*, or a *type_parameter*, and contains no instance 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 is neither a *reference_type* nor a *type_parameter* that is not constrained to be unmanaged, 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*.