A lot of doxygen updates

include/asterisk/_private.h

include/asterisk/adsi.h

include/asterisk/ael_structs.h

include/asterisk/aes.h

include/asterisk/agi.h

include/asterisk/alaw.h

include/asterisk/app.h

include/asterisk/ast_expr.h

include/asterisk/astdb.h

include/asterisk/astobj2.h

include/asterisk/callerid.h

include/asterisk/causes.h

include/asterisk/cdr.h

include/asterisk/devicestate.h

include/asterisk/doxyref.h

include/asterisk/dsp.h

include/asterisk/event.h

include/asterisk/extconf.h

include/asterisk/hashtab.h

include/asterisk/io.h