Clarify dyn vars

[holyjak]

Clarify the difference between dyn vars defined in scripts (i.e. evaluated by sci), SCI dynamic vars, and host dynamic vars.

Please answer the following questions and leave the below in as part of your PR.

• [x] I have read the developer documentation.
• [x] This PR corresponds to an issue with a clear problem statement.
• [ ] ~This PR contains a test to prevent against future regressions~

[borkdude]

Done?

[borkdude]

Thanks!