doc/examples: misc Doxygen markup improvements

Add properly formatted @example tag and fix Doxygen syntax.

doc/examples/output.c

@@ -25,8 +25,8 @@
  * libavformat API example.
  *
  * @example doc/examples/output.c
- * Output a media file in any supported libavformat format.
- * The default codecs are used.
+ * Output a media file in any supported libavformat format. The default
+ * codecs are used.
  */
 
 #include <stdlib.h>

doc/examples/transcode_aac.c

@@ -17,7 +17,10 @@
  */
 
 /**
- * @file simple audio converter
+ * @file
+ * simple audio converter
+ *
+ * @example doc/examples/transcode_aac.c
  * Convert an input audio file to AAC in an MP4 container using Libav.
  * @author Andreas Unterweger (dustsigns@gmail.com)
  */