Added info on ePin vs Pin

[walkerdanny]

Added a bit of information describing the usage of Pins and ePins. It's fairly similar to the information on the hexpansion creation page, but hopefully it clarifies things.

Also changed the mixed usage of "slot" and "port" to just be "port", and a couple of other fixes for clarity.

[npentrel]

This is great - thank you so much for writing that - it's helped at least me understand the difference 😊