Why are existing entity reference fields not available to add to a media type? I have a number of existing entity reference fields in my site. The key one is a taxonomy reference field called field_search_terms. I would like to add this field to the Document media type. However when I go to manage fields for the media type my existing fields are not available. 

It looks like I need to create a new field and any search that mixes content and media will need to have two filters.

Why is this limitation in place? Is there an issue I can follow to learn more?
I found the resources I was looking for with help from a comment by Clive:

A field can only be used on the same entity type. There is an open issue for this in the queue. According to the issue and my recollection this is a change from Drupal 7. If so it feels like a major regression.

The documentation is also incorrect at the time of this writing.