doc: no, UseEmscripten isn't available implicitly anymore.

doc/platforms-html5.dox

@@ -438,13 +438,14 @@ bundle shader code etc., as shown in the @ref examples-textured-triangle
 example). In this case the file is compiled directly into the `*.wasm` binary,
 which is the most optimal way, but it means you need to port your code away
 from the usual filesystem APIs. Another possibility is via the
-@cmake emscripten_embed_file() @ce CMake macro, which is implicitly available
+@cmake emscripten_embed_file() @ce CMake macro, which is available
 when you build for Emscripten, coming from the [UseEmscripten.cmake](https://github.com/mosra/toolchains/blob/master/modules/UseEmscripten.cmake)
 module:
 
 @code{.cmake}
 add_executable(MyApplication main.cpp)
 if(CORRADE_TARGET_EMSCRIPTEN)
+    include(UseEmscripten)
     emscripten_embed_file(MyApplication file.dat /path/in/virtual/fs/file.dat)
 endif()
 @endcode