Update UnsafeAccessorAttribute.xml

Author: xparadoxical

xml/System.Runtime.CompilerServices/UnsafeAccessorAttribute.xml

@@ -47,7 +47,7 @@ The first argument must be passed as `ref` for instance fields and methods on st
 
 The value of the first argument is not used by the implementation for `static` fields and methods and can be `null`.
 
-The return value for an accessor to a field can be `ref` if setting of the field is desired.
+The return value for an accessor to a field must be passed as `ref`.
 
 Constructors can be accessed using <xref:System.Runtime.CompilerServices.UnsafeAccessorKind.Constructor> or <xref:System.Runtime.CompilerServices.UnsafeAccessorKind.Method>.