Document that the new sections have not been approved by an RFC process

Author: gnzlbg

reference/src/layout/enums.md

@@ -353,6 +353,8 @@ enum Enum1<T> {
 
 ### Layout of single variant enums
 
+> **NOTE**: the guarantees in this section have not been approved by an RFC process.
+
 **Single variant data-carrying*** enums without a `repr()` annotation have the
 same layout as the variant field. **Single variant fieldless** enums have the
 same layout as a unit struct.
@@ -393,6 +395,8 @@ layout as `SomeStruct`.
 
 ### Layout of multi-variant enums with one inhabited variant
 
+> **NOTE**: the guarantees in this section have not been approved by an RFC process.
+
 The layout of **multi-variant** enums with **one inhabited variant** is the same
 as that of the single-variant enum containing that same inhabited variant.