What arguments get passed with PG_FUNCTION_ARGS (with the V1 convention)?

PostgreSQL Documents all their C functions with a "V1" interface, but they don't actually show what they get,

PG_FUNCTION_INFO_V1(add_one);

Datum
add_one(PG_FUNCTION_ARGS)
{
    int32   arg = PG_GETARG_INT32(0);

    PG_RETURN_INT32(arg + 1);
}

In the above what does PG_FUNCTION_ARGS declare the function to accept? Many V1 function seem to have a fcinfo magically show up in the definition, I'm guessing it's brought in here but what is it and does anything else get brought in with this macro?