[BUG] Documentation and implementation of PCA9685 do not match

[TD-er]

The pcagpio as described in the documentation is incorrect.
I guess this might have been some copy/paste from the GPIO-expanders, but the pcagpio index can only be 0...15 according to the code.

I guess here only the documentation should be updated, as we can address the commands using the taskname as prefix, which is much more clear than some index derived from the I2C address.