I have found, that in newer versions of GCC using noreturn for main causes a warning, while OS_main does not.
I did look in Help - avr-libc reference manual and downloaded the avr-libc-user-manual-1.6.5.pdf, because The documents list a few attributes, for sure not all (e.g. OS_main,...)
Finally I found some information here: http://firstname.lastname@example.org/msg03812.html
Exists any documentation that lists and explains the attributes?