Docs on ${...} usage in override values

[UncleGedd]

Describe what should be investigated or refactored

Our docs on using ${...} in override values is incomplete. The docs currently say that you only use that syntax when referencing exported vars, but from a brief look at the code it looks like you can use any Zarf vars (not just exported)

Links to any relevant code

[UncleGedd]

closed with #675