[FEATURE] Clearly document motivate and use of `OVERRIDE_` syntax or remove the functionality

[shonfeder]

Motivated by https://github.com/informalsystems/apalache/pull/155#discussion_r449052199

This seems like a useful piece of syntax, but it's currently only mentioned in passing via an example. We may want to devote a paragraph to the semantics OVERRIDE_ specifically.

[shonfeder]

Motivated by #1794, in our meeting today we discussed whether or not this functionality is actually used and useful. I think a good way forward here is to either document what motivates this feature and why it is necessary, or else mark it for deprecation and eventual removal.

FYI @Kukovec and @konnov