Inconsistent doc build output / incomplete docstrings

[jwfraustro]

• A number of optional Parameters (especially for UWS) do not have their | None in the docstring.
• Some, like version in JobSummary do have the None, even though it was included in the docstring.
• Some of the docstrings are missing their default value (XlinkType.Simple is one example).
• Rendered HTML recognizes some types, like ExecutionPhase and provides a link, but does not recognize others, like XlinkType.

See comments from @falkben here: https://github.com/spacetelescope/vo-models/pull/13

[jwfraustro]

The last two bullet points here are caused by the fact that the XLink module is not included in the toctrees of any of the other API docs. ExecutionPhase, for example, is included under the API docs for UWS.

The xlink module could be included in the API docs if desired, but it's an XML specification not an IVOA standard. It can be added if desired.