clarify missing data marginalizations in user's guide

[bob-carpenter]

Summary:

John Hall raised a question on the forums that should be addressed directly in the doc: it should be made clear that in some cases you don't need to infer the missing data as part of the model because it naturally marginalizes. Thanks to nhurre (Discourse handle) for clarifying in that post.

Description:

Just add this discussion to the doc, citing nhurre's explanation.

Current Version:

v2.20.0