include: Add \since to all documentation that was missing it.

include/SDL3/SDL_mutex.h

@@ -118,6 +118,8 @@ extern "C" {
 /**
  * Synchronization functions which can time out return this value if they time
  * out.
+ *
+ * \since This macro is available since SDL 3.0.0.
  */
 #define SDL_MUTEX_TIMEDOUT  1