audio: explicitly document audio EOF condition

This should probably be an AO function, but since the playloop still has
some strange stuff (using the buffered_audio variable instead of calling
ao_get_delay() directly), just leave it and make it more explicit.

audio/out/ao.h

@@ -49,6 +49,11 @@ typedef struct ao_control_vol {
     float right;
 } ao_control_vol_t;
 
+// If ao_get_delay() reaches this value after ao_play() was called with the
+// AOPLAY_FINAL_CHUNK flag set, the playback core expects that the audio has
+// all been played.
+#define AO_EOF_DELAY 0.05
+
 struct ao;
 struct mpv_global;
 struct input_ctx;

player/playloop.c

@@ -1219,7 +1219,7 @@ void run_playloop(struct MPContext *mpctx)
      * buffered.
      */
     if ((mpctx->d_audio || mpctx->d_video) && !audio_left && !video_left
-        && (opts->gapless_audio || buffered_audio < 0.05)
+        && (opts->gapless_audio || buffered_audio < AO_EOF_DELAY)
         && (!mpctx->paused || was_restart)) {
         if (end_is_chapter) {
             mp_seek(mpctx, (struct seek_params){