ArduSub: document used parameters

[ES-Alexander]

Would be useful if each ArduSub version specifies the parameters it actually uses. That should avoid confusing issues where someone tries to set a documented parameter, but it doesn't do anything because it exists but isn't used in the firmware.

Would likely be quite a bit of work to determine which existing parameters are unused, but shouldn't be too bad to maintain beyond that, and would be helpful for people wanting to use ArduSub vehicles to know what is definitely supported.

• Document specifications for parameters with some form of implementation in the source code (ArduSub 4.1 Initial Docs #52)
• Determine which parameters are unused
• Create more intuitive docs based on just the used parameters

[ES-Alexander]

@Williangalvani has mentioned that we can remove the sub tag from unused parameters in ArduPilot, which then means the auto-generated output can be correct for what a given ArduSub version actually supports.

Still requires determining which parameters are used, but this is likely the best mechanism for communicating that once it's determined.

[ES-Alexander]

Some internal discussion about SERVOn_FUNCTION values here.

Key points are

• hardcoded RCINx values may as well have labels representing their function (e.g. RCIN9 -> RCIN9 Lights1Level)
• other functions need to be tested to confirm what does and doesn't work
• changes need to happen in ArduPilot repo, before parameter docs can be re-built and transferred to these docs

[ES-Alexander]

• BATT discussion
• LOG discussion