added more explanations

docs/libcurl/curl_multi_info_read.3

@@ -37,6 +37,12 @@ present in that struct and can thus be used in subsequent regular
      CURLcode result;   /* return code for transfer */
    } data;
  };
+
+When \fBmsg\fP is \fICURLMSG_DONE\fP, the message identifies a transfer that
+is done, and then \fBresult\fP contains the return code for the easy handle
+that just completed.
+
+At this point, there is no other \fBmsg\fP types defined.
 .SH "RETURN VALUE"
 A pointer to a filled-in struct, or NULL if it failed or ran out of
 structs. It also writes the number of messages left in the queue (after this