decide/document stance on partially trained workflows

[simonpcouch]

re:

https://github.com/tidymodels/workflows/blob/9e6ca982f8cd381cf4601cceb64f5ad323737476/R/post-action-tailor.R#L128-L135

In the PR thread:

ec0effa surfaces an important point; removing/updating a postprocessor from an otherwise trained workflow need not remove the preprocessor and model fits, as they won't be affected by the removal of the postprocessor. This introduces the possibility of a "partially trained" workflow, where a workflow with trained preprocessor and model but untrained postprocessor should be able to fit without issue.