Documentation on `avr-as`
The documentation, such as it is, is at http://www.nongnu.org/avr-libc/

The basic AVR syntax is pretty much the same as you will find in the Atmel datasheets (unlike the way GCC uses a different x86 assembler syntax than some other x86 compilers). You can also use avr-gcc -S file.c to see the assembly gcc generates for a file.