Require explicit extern members in extern class/object and update docs

docs/whats_new.md

@@ -249,5 +249,6 @@ Lyng now provides a public Kotlin reflection bridge and a Lyng‑first class bin
 
 - **Bridge resolver**: explicit handles for values, vars, and callables with predictable lookup rules.
 - **Class bridge binding**: declare classes/members in Lyng (marked `extern`) and bind the implementations in Kotlin before the first instance is created.
+- **Extern declaration rule**: `extern class` / `extern object` are declaration-only; all members in their bodies must be explicitly marked `extern`.
 
 See **Embedding Lyng** for full samples and usage details.