update offset_of! docs to reflect the stablization of nesting

Author: lolbinarycat

library/core/src/mem/mod.rs

@@ -1257,9 +1257,6 @@ impl<T> SizedTypeProperties for T {}
 /// Enum 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:
 ///
 /// ```