Mention identifiers cannot start with `__gen` or `__tact`

[anton-trunov]

Contracts, traits, native functions, function parameters, let-variables, foreach binders, declared primitive types, structures and messages cannot be named with identifiers that start with __gen or __tact

[novusnota]

I'm thinking of some style guide page, where such mention would fit naturally. You did come up with that idea in the main repo, which may be moved here: https://github.com/tact-lang/tact/issues/294