Documentation review: PDOs and Syncs

Hey, one thing that's been bugging me while working through the code is the odd structural differences between different drivers, especially around how PDOs and sync managers are set up.  Some devices had gigantic blocks of static sync initialization code and others had none at all, and from what I could tell it mostly didn't make any difference at all.  Getting syncs right was the hardest part of the first few drivers that I worked on, and it all mystified me.  More recently, I've just... ignored them, and everything worked fine, until I started in on the CiA 402 code.  Now I think I finally understand what is going on and wrote it up here: linuxcnc-ethercat.github.io/linuxcnc-eth.../pdos-and-syncs.html

It'd be really useful if a couple people with EtherCAT experience could give that a quick read and see if they disagree about anything.