spec: document string conversion differences
diff --git a/doc/spec.md b/doc/spec.md
index 211650e..cd275cf 100644
--- a/doc/spec.md
+++ b/doc/spec.md
@@ -4094,6 +4094,7 @@
* Floating-point literals are supported (option: `-float`).
* The `float` built-in function is provided (option: `-float`).
* Real division using `float / float` is supported (option: `-float`).
+* String interpolation supports the `[ioxXeEfFgGc]` conversions.
* `def` statements may be nested (option: `-nesteddef`).
* `lambda` expressions are supported (option: `-lambda`).
* String elements are bytes.
