More formal language about Pyodide's platform support

Author: hoodmane

peps/pep-0776.rst

@@ -172,8 +172,8 @@ macOS. Upstream tools:
 Pyodide's tools:
 
 * ``pyodide build`` can be used to cross compile Python packages to run on
-  Emscripten. Cross compilation works best on Linux, sometimes on macOS, and is
-  unsupported on Windows.
+  Emscripten. Cross compilation works best on Linux, there is experimental
+  support on macOS, and it is entirely unsupported on Windows.
 * ``pyodide venv`` can make a virtual environment that runs in Pyodide.
 * ``pytest-pyodide`` can test Python code against various JavaScript runtimes.