net/dns, types/dnstypes: update some comments, tests for DoH

Clarify & verify that some DoH URLs can be sent over tailcfg
in some limited cases.

Updates #2452

Change-Id: Ibb25db77788629c315dc26285a1059a763989e24
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_test.go

@@ -586,6 +586,18 @@ func TestManager(t *testing.T) {
 				Routes: upstreams(".", "2a07:a8c0::c3:a884"),
 			},
 		},
+		{
+			name: "nextdns-doh",
+			in: Config{
+				DefaultResolvers: mustRes("https://dns.nextdns.io/c3a884"),
+			},
+			os: OSConfig{
+				Nameservers: mustIPs("100.100.100.100"),
+			},
+			rs: resolver.Config{
+				Routes: upstreams(".", "https://dns.nextdns.io/c3a884"),
+			},
+		},
 	}
 
 	trIP := cmp.Transformer("ipStr", func(ip netip.Addr) string { return ip.String() })

net/dns/resolver/forwarder_test.go

@@ -84,6 +84,11 @@ func TestResolversWithDelays(t *testing.T) {
 			in:   q("2a07:a8c0::c3:a884"),
 			want: o("https://dns.nextdns.io/c3a884"),
 		},
+		{
+			name: "nextdns-doh-input",
+			in:   q("https://dns.nextdns.io/c3a884"),
+			want: o("https://dns.nextdns.io/c3a884"),
+		},
 	}
 
 	for _, tt := range tests {