Model persistence doc

Taking over #3084. Fixes #1332

Comments on #3084 should be addressed in this patch. 
I took the liberty and simplified the "Security and maintenance limitations" section as it felt overly verbose for me.

Feel free to comment any improvements/corrections on this
@pignacio  My phrasing is bad. Maybe rephrase it that long conjunction as a 4 items bullet list to improve readability.