trivial doc fix: remove weird phrase "syscall takes zero to five arguments"

"number of arguments, from zero to five" is wrong, because on Linux maximal number
of arguments is 6, not 5. Also, maximal number of arguments is kernel-dependent,
so let's not include it here at all.

Moreover, "Each kind of system call has a definite number of arguments" is questionable.
Think about SYS_open on Linux, which takes 2 or 3 arguments. Or SYS_clone on Linux x86_64, which
takes 2 to 5 arguments. So I propose to fully remove this sentence.

Signed-off-by: Askar Safin <safinaskar@zohomail.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>