content: usage: advanced: update joystick protocols

Author: ES-Alexander

content/usage/advanced/index.md

@@ -829,7 +829,7 @@ default each joystick can provide up to 32 axis ranges and 32 buttons.
 
 #### Joystick Protocols
 
-When mapping the functionality of a joystick button or axis, there are multiple protocols to choose from:
+When mapping the functionality of a joystick button or axis, there are multiple protocols to choose from, which can be mixed and matched across different buttons and axes:
 
 {{ easy_image(src="joystick-protocols", width=500, center=true) }}
 {{ easy_image(src="joystick-axis-mapping", width=500, center=true) }}
@@ -856,9 +856,7 @@ the last joystick input it received before losing access to the joystick.
 is operating near.
 {% end %}
 
-Button functions are determined by the autopilot firmware - e.g. in ArduSub they correspond to
-[`BTNn_FUNCTION`](https://docs.bluerobotics.com/ardusub-zola/software/autopilot/ArduSub-4.1/developers/parameters/#btnn-function-function-for-button)
-parameter values.
+Button functions are [determined by the autopilot firmware](https://ardupilot.org/sub/docs/buttons.html).
 
 As a few caveats:
 - ArduSub <= 4.1.x only supports 16 independent buttons and 4 axis ranges in its `MANUAL_CONTROL` handling
@@ -888,6 +886,11 @@ Joystick buttons can also be configured to run more general functionalities, lik
 sending a single MAVLink message. These options can be provided (or defined) using
 [Cockpit's Action system](#cockpit-actions-1).
 
+##### Data Lake Variables
+
+When it is useful for the button state to represent a value within some other functionality, the button can
+be configured to control a variable within Cockpit's [Data Lake system](#data-lake).
+
 ##### Modifier Keys
 
 Modifiers allow sacrificing one button in order to add an extra functionality slot for every non-modifier button.