bpf: Update descriptions for helpers bpf_get_func_arg[_cnt]()

Now instead of the number of arguments, the number of registers
holding argument values are stored in trampoline. Update
the description of bpf_get_func_arg[_cnt]() helpers. Previous
programs without struct arguments should continue to work
as usual.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220831152657.2078805-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>