msg: set an explicit overflow message

So client API users don't have to worry about this specifically.

Also document the overflow case. (Not sure if we really need to do
this; maybe it'd be better not to, since this just adds more noise
to the docs.)

common/msg.c

@@ -321,7 +321,7 @@ static void write_msg_to_buffers(struct mp_log *log, int lev, char *text)
                 *entry = (struct mp_log_buffer_entry) {
                     .prefix = "overflow",
                     .level = MSGL_FATAL,
-                    .text = "",
+                    .text = "log message buffer overflow\n",
                 };
             }
             mp_ring_write(buffer->ring, (unsigned char *)&entry, sizeof(entry));

libmpv/client.h

@@ -1032,7 +1032,10 @@ typedef struct mpv_event_property {
 
 typedef struct mpv_event_log_message {
     /**
-     * The module prefix, identifies the sender of the message.
+     * The module prefix, identifies the sender of the message. As a special
+     * case, if the message buffer overflows, this will be set to the string
+     * "overflow" (which doesn't appear as prefix otherwise), and the text
+     * field will contain an informative message.
      */
     const char *prefix;
     /**