docs(rotator): name the Green Heron RT-21, which was already supported

The RT-21 speaks the Hy-Gain DCU-1 command set in its default protocol
setting, and internal/rotator/dcu1 has driven that since it landed —
AP1nnn/AM1 to point, AI1 to read back, over a COM port or over TCP.
Nothing was missing except a way to find it: "Green Heron" sat in a
parenthesis after "Hy-Gain DCU-1", which an operator scanning the list
for RT-21 does not see.

So the dropdown leads with the controller people actually own, and the
hint says the two things that decide whether it works: the RT-21's
protocol must be set to DCU-1 / Rotor-EZ (on GS-232 it belongs to the
GS-232 entry instead), and with the Ethernet option TCP reaches it
directly rather than through a serial-over-IP bridge, which is what the
hint used to imply was necessary.

The package doc now also records what is NOT here: the native Green
Heron protocol is a third command set, with 0.1° readback and a real
stop, and writing it from memory is exactly the mistake this repo warns
about for wire protocols.

internal/rotator/dcu1/dcu1.go

@@ -1,11 +1,18 @@
 // Package dcu1 drives rotator controllers that speak the Hy-Gain DCU-1 protocol,
 // over a serial COM port (or a raw TCP socket, e.g. a serial-over-IP bridge).
 //
-// DCU-1 is used by the Hy-Gain DCU-1, the Idiom Press Rotor-EZ, Green Heron
-// controllers, and the RotorCard DXA (hamsupply) for Yaesu DXA rotors. It is a
+// DCU-1 is used by the Hy-Gain DCU-1, the Green Heron RT-21, the Idiom Press
+// Rotor-EZ, and the RotorCard DXA (hamsupply) for Yaesu DXA rotors. It is a
 // DIFFERENT command set from Yaesu GS-232 (see internal/rotator/gs232):
 // semicolon-terminated, azimuth only.
 //
+// The RT-21 selects its protocol on the controller, and only its DCU-1 /
+// Rotor-EZ setting is this one — an RT-21 left on GS-232 belongs to the gs232
+// package instead. With the Ethernet option it is a TCP endpoint in its own
+// right, so the TCP transport below reaches it without a serial-over-IP
+// bridge. Its NATIVE Green Heron protocol is a third command set, with 0.1°
+// readback and a real stop, and is not implemented here.
+//
 // Commands (';' terminated — roundTrip appends the ';'):
 //
 //	AP1nnn   set the target bearing nnn (000-359)