wasm: final review nits

Drop the wasm example's redundant header comment and default_visibility
(nothing outside the package references its targets), and document why the
SDK repository pairs a rules_cc toolchain with the Swift one: it drives the
bundle's own clang, resolves only for the wasm target platform, and a
consumer's root-module registration takes precedence over it.

Author: AttilaTheFun

examples/cross_compilation/wasm/BUILD.bazel

@@ -1,12 +1,7 @@
-# A WebAssembly reactor written in Swift, executed under a hermetic wasmtime
-# and embedded in a static web app.
-
 load("@rules_shell//shell:sh_test.bzl", "sh_test")
 load("//examples/embedded:transition.bzl", "transition_binary")
 load("//swift:swift.bzl", "swift_binary", "swift_library")
 
-package(default_visibility = ["//visibility:public"])
-
 platform(
     name = "wasm32-wasip1",
     constraint_values = [

swift/internal/extensions/toolchains.bzl

@@ -66,6 +66,12 @@ toolchain(
     visibility = ["//visibility:public"],
 )
 
+# The paired rules_cc toolchain drives the SDK bundle's own clang for the C
+# side of the build (unlike Android, where the cc toolchain comes from the
+# separately registered NDK). It only resolves for this wasm target platform,
+# and because root-module registrations take precedence in toolchain
+# resolution, a consumer who registers their own wasm cc toolchain wins over
+# this one automatically.
 toolchain(
     name = "cc_toolchain_{target}_{platform}",
     exec_compatible_with = {exec_compatible_with},