Document how to use connectors from platform definitions

[jfng]

This question comes up regularly on the #nmigen IRC channel.

[awygle]

In the meantime, in case anyone (like me) wants to help someone out with this in the future and can't remember how it's supposed to work, here's ktemkin explaining it in IRC: https://freenode.irclog.whitequark.org/nmigen/2020-09-24#27984233;

[awygle]

Also, as part of this documentation, it would be great to include a brief explanation of why the system is the way it is - I think it's pretty counterintuitive to new users coming from Verilog in particular.