Corrections and updates to configuration_users.rst

Please review for technical accuracy and typos
I don't think that any SQL statement would be safe.

It seems there is no good way to keep a share. The only way I can think of is doing the following:
1) Let's say user "to_delete" shared a folder "shared_files" with everyone, including the new "permanent_user"
2) "permanent_user" user creates a new folder "new_shared_files"
3) "permanent_user" moves all files from "shared_files" into to "new_shared_files" (drag and drop in web UI, or WebDAV, or sync client)
4) "admin" deletes "to_delete" which makes the folder "shared_files" disappear for everyone
5) "permanent_user" renamed "new_shared_files" to "shared_files"
6) "permanent_user" shares "shared_files" with everyone the other folder was shared with