sensor: add note about passkey secrecy

Author: lopsided98

sensor/firmware/prj.conf

@@ -35,6 +35,9 @@ CONFIG_BT_LONG_WQ=y
 # (and being able to fix problems remotely) than security.
 # [0] https://insinuator.net/2021/10/change-your-ble-passkey-like-you-change-your-underwear/
 CONFIG_BT_FIXED_PASSKEY=y
+# Given that the fixed passkey provides minimal security, there is no harm in
+# disclosing it in the public repo. It is just intended to keep out curious
+# passersby.
 CONFIG_BT_DEFAULT_PASSKEY=207943
 CONFIG_BT_SETTINGS=y