doc language - cs3a.md

The following changes attempt to add more detail to the explanations for cipher set 3a.

A few more details were added to the switch/hashname, open, and line sections.

The example code was split in to sender/receiver examples and more detailed doc strings were added.  This hopefully clarifies the open handshake from an implementation perspective while also still explaining the overall scheme.

More examples for sending post-open line packets could be added, but this seems like a good place to get some feedback.

Your time for consideration, review, and corrections is appreciated.

thanks,
cody