Update documentation for key: requires

[minhio]

Chatted with Mike Barrett on Slack, I had a question about the requires key and what you do not need to set this if you refer to another stack in a Parameter meant.

Mike's response

re: the Parameter thing - that’s a “documentation bug” or just leftover docs that should be updated, can you open an issue? Basically nowadays the requires section is optional IF you use ${output} lookups in variables, because that automatically builds relationships for you.