Update supported file extensions in doxygen.

Updated the doxygen \arg line after looking at the file for some other Asterisk documentation
and noticing they weren't up to date. Thanks to seanbright for looking at the code for me :)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@257988 65c4cc65-6c06-0410-ace0-fbb531ad65f3

formats/format_pcm.c

@@ -19,7 +19,7 @@
 /*! \file
  *
  * \brief Flat, binary, ulaw PCM file format.
- * \arg File name extension: pcm, ulaw, ul, mu
+ * \arg File name extension: alaw, al, alw, pcm, ulaw, ul, mu, ulw, g722, au
  * 
  * \ingroup formats
  */