Clarified restrictions

README.md

@@ -189,10 +189,11 @@ While data structures with any of these attributes should generally roundtrip th
 - struct names inside an internally (or adjacently) tagged or untagged enum, e.g. by enabling the `PrettyConfig::struct_types` setting, are not supported
 - enabling the `#![enable(implicit_some)]` extension on a document with internally (or adjacently) tagged or untagged enums is not supported
 - untagged tuple / struct variants with no fields are not supported
-- tuple variants with just one field (that are not newtype variants) are not supported
-- internally tagged newtype variants must not contain a value that looks like a unit `()` in ron but isn't one, e.g. an untagged unit variant
+- untagged tuple variants with just one field (that are not newtype variants) are not supported when the `#![enable(unwrap_variant_newtypes)]` extension is enabled
+- internally tagged newtype variants must not contain a unit / unit struct inside an untagged newtype variant, or an untagged unit variant
 - serde does not yet support `i128` and `u128` inside internally (or adjacently) tagged or untagged enums
-- newtypes and zero-length arrays / tuple structs / structs / tuple variants / struct variants are not supported inside internally (or adjacently) tagged or untagged enums
+- newtypes and zero-length arrays / tuples / tuple structs / structs / tuple variants / struct variants are not supported inside internally (or adjacently) tagged or untagged enums
+- externally tagged tuple variants with just one field (that are not newtype variants) are not supported inside internally (or adjacently) tagged or untagged enums
 
 Please file a [new issue](https://github.com/ron-rs/ron/issues/new) if you come across a use case which is not listed among the above restrictions but still breaks.