doc, win: improve os.setPriority documentation

PR-URL: https://github.com/nodejs/node/pull/22817
Fixes: https://github.com/nodejs/node/issues/22799
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/os.md

@@ -371,6 +371,9 @@ priority classes, `priority` is mapped to one of six priority constants in
 mapping may cause the return value to be slightly different on Windows. To avoid
 confusion, it is recommended to set `priority` to one of the priority constants.
 
+On Windows setting priority to `PRIORITY_HIGHEST` requires elevated user,
+otherwise the set priority will be silently reduced to `PRIORITY_HIGH`.
+
 ## os.tmpdir()
 <!-- YAML
 added: v0.9.9

test/parallel/test-os-process-priority.js

@@ -116,8 +116,10 @@ function checkPriority(pid, expected) {
     return;
   }
 
+  // On Windows setting PRIORITY_HIGHEST will only work for elevated user,
+  // for others it will be silently reduced to PRIORITY_HIGH
   if (expected < PRIORITY_HIGH)
-    assert.strictEqual(priority, PRIORITY_HIGHEST);
+    assert.ok(priority === PRIORITY_HIGHEST || priority === PRIORITY_HIGH);
   else if (expected < PRIORITY_ABOVE_NORMAL)
     assert.strictEqual(priority, PRIORITY_HIGH);
   else if (expected < PRIORITY_NORMAL)