Heartbeat documentation is not clear that it does not apply to Bluetooth - Add BT instructions

[gregpakes]

iOS SDK v1.9.2, X4.

In the README, "Send Heartbeat Packets to the Camera" sits inside Connection
Module directly after "Connect the Camera via Bluetooth", so it reads as
something a Bluetooth integration has to do. The header says otherwise:

/*!
 * Call this method as soon as Socket connection has be connected
 *
 * availability(ONE X, Go)
 */
- (void)sendHeartbeatsWithOptions:(nullable INSHeartbeatsSenderOptions *)options;

Your own Bluetooth sample matches the header rather than the docs — it starts
heartbeats only when usb(), external() or socket() reaches .connected,
and BluetoothViewController never sends one.

The section is easy to act on wrongly because it names symptoms a BLE
integration will actually be seeing ("interrupted previews or invalid
commands", "the camera will actively disconnect"). We sent
sendHeartbeatsWithOptions: on the Bluetooth command manager for weeks. It's
accepted there — INSAllBluetoothCommands inherits it — so it compiles, runs,
and silently does nothing, while we chased a link that drops ~59s after
connect as if it were a heartbeat problem.

Could the heading say Wi-Fi/USB, or move under the Wi-Fi section? And if
something is meant to keep a Bluetooth link alive, it'd be worth saying what.
Your Bluetooth sample polls getCurrentCaptureStatusWithCompletion: every 5s
in captureStatusForever(), which is marked availability(ONE bluetooth, ONE X) — if that's the intended answer, that isn't obvious from the docs.