Point out corner case of all()

This is probably better said here than in traps.pod

lib/Type/Junction.pod

@@ -61,6 +61,14 @@ Usage examples:
     my @primes_ending_in_1 = grep &is_prime & / 1$ /, 2..100;
     say @primes_ending_in_1;        # 11 31 41 61 71
 
+Special care should be taken when using C<all> with arguments that may
+produce an empty list:
+
+    my @a = ();
+    so all(@a) # True, because there are 0 False's
+    so all(@a, ?@a) # Does what you might have meant by the above
+    so all(@a, True) # Explicit way to express the first example
+    
 Negated operators are special-cased when it comes to autothreading.
 C<$a !op $b> is rewritten internally as C<!($a op $b)>. The outer
 negation collapses any junctions, so the return value always a plain