doc: contrib: clarify twister support

Twister works on all OSes with different levels of features/targets
supported, so clarify the message.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

doc/contribute/guidelines.rst

@@ -573,7 +573,7 @@ twister
 
 .. note::
    twister is only fully supported on Linux; on Windows and MacOS the execution
-   of tests is not supported, only building.
+   of tests is not supported on all target devices.
 
 If you think your change may break some test, you can submit your PR as a draft
 and let the project CI automatically run the :ref:`twister_script` for you.