add note about example

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5872 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk.h

@@ -91,6 +91,11 @@ void ast_unregister_file_version(const char *file);
  * \code
  * ASTERISK_FILE_VERSION(__FILE__, "\$Revision\$")
  * \endcode
+ *
+ * \note The dollar signs above have been protected with backslashes to keep
+ * CVS from modifying them in this file; under normal circumstances they would
+ * not be present and CVS would expand the Revision keyword into the file's
+ * revision number.
  */
 #ifdef __GNUC__
 #define ASTERISK_FILE_VERSION(file, version) \