Improve documentation for required values

[eddie-knight]

Currently the format for required values is somewhat lacking, as it requires a Ctrl+F to find all the required fields. Then some fields may easily be mis-interpreted as required when they are conditionally required for a block (the block isn't required, but if included then certain keys are subsequently required).

[eddie-knight]

This has been somewhat improved in the updated documentation via #96. We should revisit this if anyone is still having difficulty with the clarity around the required fields.