qemu-thread: Document QemuEvent

Document QemuEvent to help choose an appropriate synchronization
primitive.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Link: https://lore.kernel.org/r/20250529-event-v5-12-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/thread.h

@@ -10,6 +10,16 @@ typedef struct QemuSemaphore QemuSemaphore;
 typedef struct QemuLockCnt QemuLockCnt;
 typedef struct QemuThread QemuThread;
 
+/*
+ * QemuEvent
+ * =========
+ *
+ * QemuEvent is an implementation of Win32 manual-reset event object.
+ * For details, refer to:
+ * https://learn.microsoft.com/en-us/windows/win32/sync/using-event-objects
+ *
+ * QemuEvent is more lightweight than QemuSemaphore when HAVE_FUTEX is defined.
+ */
 typedef struct QemuEvent {
 #ifndef HAVE_FUTEX
     pthread_mutex_t lock;