bpo-38317: Fix PyConfig.warnoptions priority (GH-16478)

Fix warnings options priority: PyConfig.warnoptions has the highest
priority, as stated in the PEP 587.

* Document options order in PyConfig.warnoptions documentation.
* Make PyWideStringList_INIT macro private: replace "Py" prefix
  with "_Py".
* test_embed: add test_init_warnoptions().
diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index d5effb8..8ce5622 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -220,7 +220,10 @@
     wchar_t *program_name;
 
     PyWideStringList xoptions;     /* Command line -X options */
-    PyWideStringList warnoptions;  /* Warnings options */
+
+    /* Warnings options: lowest to highest priority. warnings.filters
+       is built in the reverse order (highest to lowest priority). */
+    PyWideStringList warnoptions;
 
     /* If equal to zero, disable the import of the module site and the
        site-dependent manipulations of sys.path that it entails. Also disable