update offset_of! docs to reflect the stablization of nesting

Author: lolbinarycat

core/src/mem/mod.rs

@@ -1254,12 +1254,10 @@ impl<T> SizedTypeProperties for T {}
 ///
 /// Nested field accesses may be used, but not array indexes.
 ///
-/// Enum variants may be traversed as if they were fields. Variants themselves do
+/// If the nightly-only feature `offset_of_enum` is enabled,
+/// variants may be traversed as if they were fields. Variants themselves do
 /// not have an offset.
 ///
-/// However, on stable only a single field name is supported, which blocks the use of
-/// enum support.
-///
 /// Visibility is respected - all types and fields must be visible to the call site:
 ///
 /// ```