client API: clarify mpv_get_wakeup_pipe docs

It wasn't necessarily clear how this works. Especially make clear that
the API user shouldn't expect that there's one byte per readable event
in the wakeup pipe. Actually, the reason why this currently won't work
is because property notifications can generate more events than wakeups.
The limit of 4096 is a more fundamental issue, but the event ringbuffer
is currently limited to 1000 entries anyway.

Also add some important comments to mpv_set_wakeup_callback.

libmpv/client.h

@@ -1179,7 +1179,10 @@ void mpv_wakeup(mpv_handle *ctx);
  *
  * In general, the client API expects you to call mpv_wait_event() to receive
  * notifications, and the wakeup callback is merely a helper utility to make
- * this easier in certain situations.
+ * this easier in certain situations. Note that it's possible that there's
+ * only one wakeup callback invocation for multiple events. You should call
+ * mpv_wait_event() with no timeout until MPV_EVENT_NONE is reached, at which
+ * point the event queue is empty.
  *
  * If you actually want to do processing in a callback, spawn a thread that
  * does nothing but call mpv_wait_event() in a loop and dispatches the result
@@ -1197,7 +1200,8 @@ void mpv_set_wakeup_callback(mpv_handle *ctx, void (*cb)(void *d), void *d);
  * pipe can be used to wake up a poll() based processing loop. The purpose of
  * this function is very similar to mpv_set_wakeup_callback(), and provides
  * a primitive mechanism to handle coordinating a foreign event loop and the
- * libmpv event loop.
+ * libmpv event loop. The pipe is non-blocking. It's closed when the mpv_handle
+ * is destroyed. This function always returns the same value (on success).
  *
  * This is in fact implemented using the same underlying code as for
  * mpv_set_wakeup_callback() (though they don't conflict), and it is as if each
@@ -1205,17 +1209,43 @@ void mpv_set_wakeup_callback(mpv_handle *ctx, void (*cb)(void *d), void *d);
  * becomes readable, the code calling poll() (or select()) on the pipe should
  * read all contents of the pipe and then call mpv_wait_event(c, 0) until
  * no new events are returned. The pipe contents do not matter and can just
- * be discarded.
+ * be discarded. There is not necessarily one byte per readable event in the
+ * pipe. For example, the pipes are non-blocking, and mpv won't block if the
+ * pipe is full. Pipes are normally limited to 4096 bytes, so if there are
+ * more than 4096 events, the number of readable bytes can not equal the number
+ * of events queued. Also, it's possible that mpv does not write to the pipe
+ * once it's guaranteed that the client was already signaled. See the example
+ * below how to do it correctly.
  *
- * Note that this call lazily creates the pipe, and always returns the same
+ * Example:
- * handle once it's created. The client API will destroy both the read and
- * write ends of the pipe in mpv_destroy(). If you need something more
- * complex, it's better to implement your own mechanisms using
- * mpv_set_wakeup_callback().
  *
- * On Windows, this will always return -1.
+ *  int pipefd = mpv_get_wakeup_pipe(mpv);
+ *  if (pipefd < 0)
+ *      error();
+ *  while (1) {
+ *      struct pollfd pfds[1] = {
+ *          { .fd = pipefd, .events = POLLIN },
+ *      };
+ *      // Wait until there are possibly a new mpv events.
+ *      poll(pfds, 1, -1);
+ *      if (pfds[0].revents & POLLIN) {
+ *          // Empty the pipe. Doing this before calling mpv_wait_event()
+ *          // ensures that no wakeups get missed.
+ *          char unused[256];
+ *          read(pipefd, unused, sizeof(unused));
+ *          while (1) {
+ *              mpv_event *ev = mpv_wait_event(mpv, 0);
+ *              // If MPV_EVENT_NONE is received, the event queue is empty.
+ *              if (ev->event_id == MPV_EVENT_NONE)
+ *                  break;
+ *              // Process the event.
+ *              ...
+ *          }
+ *      }
+ *  }
  *
- * @return A UNIX FD of the read end of the wakeup pipe, -1 on error.
+ * @return A UNIX FD of the read end of the wakeup pipe, or -1 on error.
+ *         On MS Windows/MinGW, this will always return -1.
  */
 int mpv_get_wakeup_pipe(mpv_handle *ctx);