lua-howto: document the Administered bind storage mode

[MUSTARDTIGERFPV]

The Bind Storage section lists Persistent, Volatile and Returnable, but not Administered. It is the fourth entry in the Lua selection (RXParameters.cpp, "Persistent;Volatile;Returnable;Administered") and in the WebUI binding panel, and it has shipped since 3.5.4.

What it does

Enforcement is in one place, EnterBindingMode() in rx_main.cpp:

// never enter binding mode if binding is supposed to only be administered through the web UI
if (config.GetBindStorage() == BINDSTORAGE_ADMINISTERED) {
    return;
}

Every route into bind mode goes through that function, so all of these are blocked:

Route	Call site
Three power-cycle	GetPowerOnCounter() >= 3
Automatic bind when unbound	!OtaUidIsBound(UID)
Enter Bind Mode command	RXEndpoint.cpp via EnterBindingModeSafely()
Bind button action	registerButtonFunction(ACTION_BIND, ...)

The WebUI path is not gated. devWIFI.cpp only tests bind storage for Volatile and Returnable, so the WebUI remains the only way to change the binding.

The warning

An unbound receiver set to Administered will not bind over the air either, because the automatic bind path also goes through EnterBindingMode(). Recovery is the WebUI, or a UART re-flash if the WebUI cannot be reached. The added warning says so and suggests setting a Home WiFi network first.

Version

git tag --contains on both the original BINDSTORAGE_PERMANENT commit and the later rename to BINDSTORAGE_ADMINISTERED returns 3.5.4 as the first release containing them.