Document FieldInfo.SetValue breaking change (dotnet#3869)

* Document FieldInfo.SetValue breaking change

* Apply suggestions from code review

Co-Authored-By: Genevieve Warren <[email protected]>

* Apply suggestions from code review

Add comma

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

Author: jkotas

xml/System.Reflection/FieldInfo.xml

@@ -2102,6 +2102,8 @@
 ## Remarks  
  This method will assign `value` to the field reflected by this instance on object `obj`. If the field is static, `obj` will be ignored. For non-static fields, `obj` should be an instance of a class that inherits or declares the field. The new value is passed as an `Object`. For example, if the field's type is Boolean, an instance of `Object` with the appropriate Boolean value is passed. Before setting the value, `SetValue` checks to see if the user has access permission. This final method is a convenience method for calling the following `SetValue` method.  
   
+This method cannot be used to set values of static, init-only (`readonly` in C#) fields reliably. In .NET Core 3.0 and later versions, an exception is thrown if you attempt to set a value on a static, init-only field.
+
 > [!NOTE]
 >  Fully trusted code has the permissions that are needed to access and invoke private constructors, methods, fields, and properties using reflection.  
   
@@ -2200,6 +2202,8 @@
   
 ## Remarks  
  This method will assign `value` to the field reflected by this instance on `obj`. If the field is static, `obj` will be ignored. For non-static fields, `obj` should be an instance of a class that inherits or declares the field. The new value is passed as an `Object`. For example, if the field's type is `Boolean`, an instance of `Object` with the appropriate Boolean value is passed. Before setting the value, `SetValue` checks to see if the user has access permission.  
+
+This method cannot be used to set values of static, init-only (`readonly` in C#) fields reliably. In .NET Core 3.0 and later versions, an exception is thrown if you attempt to set a value on a static, init-only field.
   
 > [!NOTE]
 >  Fully trusted code has the permissions that are needed to access and invoke private constructors, methods, fields, and properties using reflection.