mp4: update comments around fix

Now that this MP4Read fix is practically finalized, make sure we properly document why we make this call.

ip/mp4.c

@@ -172,9 +172,13 @@ static int mp4_open(struct input_plugin_data *ip_data)
 
 	/* open mpeg-4 file */
 #if MP4V2_PROJECT_version_major < 2 ||  (MP4V2_PROJECT_version_major == 2 && MP4V2_PROJECT_version_minor < 1)
-	/* MP4Read doesnt have a second argument on macos */
 	priv->mp4.handle = MP4Read(ip_data->filename);
 #else
+	/* some distributions [notably arch linux] use techsmith's mp4v2 fork instead of the largely abandoned
+	   upstream project. This fork however is primarily designed for internal techsmith use, and in 4.1.4/4.1.5
+	   they added an extra argument to MP4Read that when dynamically called my cmus would result in a segfault.
+	   to fix this, we check if we are using the techsmith fork by seeing if the version is higher than the last
+	   upstream version, and then specify the argument in that case. */
 	priv->mp4.handle = MP4Read(ip_data->filename, NULL);
 #endif