win, doc: improve uv_os_setpriority documentation

Ref: nodejs/node#22817

Author: bzoz

docs/src/misc.rst

@@ -544,4 +544,7 @@ API
         process priority, the result will equal one of the `UV_PRIORITY`
         constants, and not necessarily the exact value of `priority`.
 
+        Setting PRIORITY_HIGHEST will only work for elevated user, for others it
+        will be silently reduced to PRIORITY_HIGH
+
     .. versionadded:: 1.23.0