Clarification of type for `return_code` in output section

[vsmalladi]

Currently the spec reads:

return_code: An Int? whose value is initially None and is set to the value of the command's return code. The value is only guaranteed to be defined in the output section.

Does this mean that its expected to be Int? in the output section or if it can just be an Int.

[claymcleod]

Relevant comment: #702 (comment).