Things to explain in the docs

[nedbat]

Based on questions people asked when I announced scriv, these things need explaining:

• "Do you have a demo of how fragment categories are selected?"

[webknjaz]

Also:
2) The docs mention the ability to write change notes for different audiences. How is this achieved? Do I understand correctly that I can write the same text twice but with different accents: for example, one for devs (like private API and/or implementation changes), and the other for end-users (or lib users, like public API changes)

[webknjaz]

Another one:
3) There's this problem that surfaced with towncrier: we've changed the template to include category names and since they were missing, and now only new releases will use that template so we have to find a way to retrospectively extract the fragments to reconstruct the changelog with a new template (pypa/setuptools#2399 (comment)). What if I change the template in scriv and want to update how the changelog looks? Is there any recommended/suppoprted way of doing that? I noticed there's --keep but the docs don't exactly describe what will happen if I keep the fragments in the tree.