STYLE.md: say more about edition annotations for examples

STYLE.md

@@ -41,7 +41,7 @@ See https://highlightjs.org/ for a list of supported languages.
 
 Rust examples are tested via rustdoc, and should include the appropriate annotations when tests are expected to fail:
 
-* `edition2018` — If it is edition-specific.
+* `edition2015` or `edition2018` — If it is edition-specific (see `book.toml` for the default).
 * `no_run` — The example should compile successfully, but should not be executed.
 * `should_panic` — The example should compile and run, but produce a panic.
 * `compile_fail` — The example is expected to fail to compile.