block steps: Added note about different format for different numbers of option fields.
block steps: Added note about different format for different numbers of option fields.

This would be nice to have on docs (: Unless there is more to it, than I just found.

However, I need from you this magic `X` number. 

Let me know if that makes sense.
buildkite/docs,buildkite/docs,buildkite/docs,buildkite/docs