documenting outputs in **parameter**_meta isn't clear

[a-frantz]

The current situation where both inputs and outputs are documented within the parameter_meta section can be confusing. I've been told this overloading can be traced back to this issue on the cromwell repo. I would like to see the addition of an output_meta section to the specification. It would function similarly to the current parameter_meta, except only declared outputs would be permitted within.

Since inputs and outputs share a namespace, it could also make sense to keep the current behavior of parameter_meta but rename it to something like declaration_meta. IMO having two separate sections, one for inputs and one for outputs is the cleaner solution which will lead to better documentation.