Again, the function's description is misleading right now. I sought a function, which (mathematically) computes A - B, or, written differently, A \ B. Or, again in other words, suppose
A := {a1, ..., an} and B:= {a1, b1, ... , bm}
=> array_diff(A,B) = {a2, ..., an}
array_diff(A,B) returns all elements from A, which are not elements of B (= A without B).
You should include this in the documentation more precisely, I think.