Ideally, the description for each admin setting should provide enough information right there on the page itself (and we’re always open to suggestions about how we can make those clearer if needed 
). But meta.discourse.org is the de facto Discourse user manual. 
If you can’t find anything in Search then feel free to create a new topic and ask what you need in there.
I don’t believe there is an easy way without using the rails console. You can individually Unpin and then Pin each topic again using the topic wrench, however, a user can unpin and repin a topic as and when they like, so even if you reset them all they can still be overridden. If you have some info that you want to always be visible you could try using the Versatile Banner theme component?