Mention single thread web template support (#9392)

* Mention single thread web template support

This is controlled  with the SCons option ``threads``. Now it's documented as another knob you can turn when compiling the web template.

* Update contributing/development/compiling/compiling_for_web.rst

Co-authored-by: A Thousand Ships <[email protected]>

---------

Co-authored-by: Max Hilbrunner <[email protected]>
Co-authored-by: A Thousand Ships <[email protected]>

contributing/development/compiling/compiling_for_web.rst

@@ -51,6 +51,12 @@ enabled. Since ``eval()`` calls can be a security concern, the
     scons platform=web target=template_release javascript_eval=no
     scons platform=web target=template_debug javascript_eval=no
 
+By default, WebWorker threads support is enabled. To disable it and only use a single thread,
+the ``threads`` option can be used to build the web template without threads support::
+
+    scons platform=web target=template_release threads=no
+    scons platform=web target=template_debug threads=no
+
 The engine will now be compiled to WebAssembly by Emscripten. Once finished,
 the resulting file will be placed in the ``bin`` subdirectory. Its name is
 ``godot.web.template_release.wasm32.zip`` for release or ``godot.web.template_debug.wasm32.zip``