docs: Clarify the rationale for split trait implementations for native and WASM targets

evm_client/tests/wasm.rs

@@ -1,3 +1,5 @@
+// This test file is only compiled for wasm32. The DummySigner uses #[async_trait::async_trait(?Send)]
+// because WASM async traits do not require Send. See balance.rs or evm_client.rs for the trait bound split rationale.
 #![cfg(target_arch = "wasm32")]
 use wasm_bindgen_test::*;
 wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);