Improvements to the JavaDocs in the API

common/src/main/java/me/lucko/luckperms/api/implementation/internal/UserLink.java

@@ -66,7 +66,7 @@ public class UserLink extends PermissionHolderLink implements User {
     }
 
     @Override
-    public void setPrimaryGroup(String s) throws ObjectAlreadyHasException {
+    public void setPrimaryGroup(@NonNull String s) throws ObjectAlreadyHasException {
         if (getPrimaryGroup().equalsIgnoreCase(s)) {
             throw new ObjectAlreadyHasException();
         }

common/src/main/java/me/lucko/luckperms/api/implementation/internal/Utils.java

@@ -33,19 +33,19 @@ class Utils {
 
     static void checkUser(User user) {
         if (!(user instanceof UserLink)) {
-            throw new IllegalArgumentException("User instance cannot be handled by this implementation.");
+            throw new IllegalStateException("User instance cannot be handled by this implementation.");
         }
     }
 
     static void checkGroup(Group group) {
         if (!(group instanceof GroupLink)) {
-            throw new IllegalArgumentException("Group instance cannot be handled by this implementation.");
+            throw new IllegalStateException("Group instance cannot be handled by this implementation.");
         }
     }
 
     static void checkTrack(Track track) {
         if (!(track instanceof TrackLink)) {
-            throw new IllegalArgumentException("Track instance cannot be handled by this implementation.");
+            throw new IllegalStateException("Track instance cannot be handled by this implementation.");
         }
     }