Clarify that intel only installer can create universal2 wheels

osx_utils.sh

@@ -159,6 +159,10 @@ function pyinst_fname_for_version {
     # Use the universal2 installer if we are on arm64
     # universal2 installer for python 3.8 needs macos 11.0 to run on
     # and therefore x86_64 builds use the intel only installer.
+    # Note that intel only installer can create universal2 wheels, but
+    # creates intel only wheels by default. When PLAT=universal2
+    # we set the env variable _PYTHON_HOST_PLATFORM to change this
+    # default.
     if [ "$(uname -m)" == "arm64" ]; then
       if [ "$py_version" == "3.9.1" ]; then
         echo "python-${py_version}-macos11.0.${inst_ext}"