test/system: Improve TODO comments on IPv6 pasta custom DNS forward test

This test is currently disabled due to several issues, only some of which
are described in the existing comments.  Add some more details to clarify
the situation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

test/system/505-networking-pasta.bats

@@ -462,7 +462,12 @@ function pasta_test_do() {
 @test "Custom DNS forward address, IPv6" {
     skip_if_no_ipv6 "IPv6 not routable on the host"
 
-    # TODO: Two issues here:
+    # TODO: In fact, this requires not just IPv6 connectivity on the
+    #       host, but an IPv6 reachable nameserver which is harder to
+    #       test for.  We could remove that requirement if pasta could
+    #       forward between IPv4 and IPv6 addresses but as of
+    #       2024_09_06.6b38f07 that's unsupported.  Skip the test for
+    #       now.
     skip "Currently unsupported"
     # local addr=2001:db8::1
     #
@@ -472,6 +477,10 @@ function pasta_test_do() {
     # run_podman run --rm --net=pasta:--dns-forward,$addr \
     #     $IMAGE nslookup l.root-servers.net $addr
     #
+    # TODO: In addition to the IPv6 nameserver requirement above,
+    #       there seem to be two problems running this test.  It's
+    #       unclear if those are in busybox, musl or pasta.
+    #
     # 1. With this, Podman writes "nameserver 2001:db8::1" to
     #    /etc/resolv.conf, without zone, and the query originates from ::1.
     #    Passing: