Adding Tilghman's documentation from asterisk-dev to the actual file.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@239389 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/astmm.h

@@ -18,6 +18,9 @@
 
 /*! \file
  * \brief Asterisk memory usage debugging
+ * This file provides headers for MALLOC_DEBUG, a define used for tracking down
+ * memory leaks.  It should never be #included directly; always use the
+ * MALLOC_DEBUG definition in menuselect to activate those functions.
  */