core: Add some annotations to managed thread variables

Some thread variables don't need to be set manually,
since they are updated whenever needed by some functions.
Add comments to make that clear.

core/src/thread.zig

@@ -17,12 +17,13 @@ pub const ThreadControlBlock = struct {
     mapper: ?vmm.MemoryMapper,
     regs: interrupts.InterruptStackFrame,
     state: ThreadState,
-
-    ticks: u64,
-
     user_priority: u8,
-    current_priority: u32,
 
+    // Managed by scheduleNewTask(), no need to set manually.
+    ticks: u64,
+    // Managed by addThreadToPriorityQueue(), no need to set manually.
+    current_priority: u32,
+    // Managed by startSleep(), no need to set manually.
     sleep_ticks: u64,
 };