Add expected failure description to test/known_problems/should_fail/t…

…uple_union_arg.erl

test/known_problems/should_fail/tuple_union_arg.erl

@@ -6,5 +6,12 @@
        (d, e) -> {d, e}.
 i(V, U) -> {V, U}.
 
+%% This passes, though it shouldn't, because V is inferred to be d | a,
+%% and U is inferred to be b | e.
+%% If that was the case, then the call to i/2 could sometimes succeed.
+%% Not always, though! So it should already be considered an error.
+%%
+%% However, due to how the union is structured we know that when V=d, then U=b,
+%% and that combination is certain to fail. The same holds for V=a, U=e.
 -spec j({d, b} | {a, e}) -> {a, b} | {d, e}.
 j({V, U}) -> i(V, U).