Give examples for changing-state-fns-with-exclamation-mark

[danielcompton]

Reading https://github.com/bbatsov/clojure-style-guide#changing-state-fns-with-exclamation-mark, it's not clear to me whether things like database calls should have !s on them. They are changing state, and not necessarily retryable in STM transactions, but it's not super clear what the boundaries are here. It would be good to put some concrete examples here of what is in and what is out.

e.g.

In
STM
Database mutations
Saving files to disk

Out
Logging
???