Submissions for closing or closed forms in Central can't be edited

Currently, it's not possible to edit submissions if the form is marked as closed or closing. For workflows that require making sure data collectors can't fill out forms anymore after a point in time but still allowing edits, document options of removing app user access, deleting app user or deleting public link. There is (I believe) no workaround for projects in which Web Users fill out forms from Central. They'd need to be told not to fill those forms out anymore.

No specific file or test is named. Start in the Sphinx documentation covering Central forms and submission editing, then verify that it explains how to stop new data collection while preserving edits for the workflows described. Done means the documented options and the Web Users limitation are clear.

Written by the indexing model from the issue text.