Add possibility for disabling inlining wasm in opencv.js

doc/js_tutorials/js_setup/js_setup/js_setup.markdown

@@ -83,6 +83,9 @@ Building OpenCV.js from Source
     It requires `python` and `cmake` installed in your development environment.
 
 -#  The build script builds asm.js version by default. To build WebAssembly version, append `--build_wasm` switch.
+    By default everything is bundled into one JavaScript file by `base64` encoding the WebAssembly code. For production
+    builds you can add `--disable_single_file` which will reduce total size by writing the WebAssembly code
+    to a dedicated `.wasm` file which the generated JavaScript file will automatically load.
 
     For example, to build wasm version in `build_wasm` directory:
     @code{.bash}