Dissuade deprecated slash syntax

On [the Expressions page], slash syntax for object paths is said to be deprecated, but suggests that you may still use it as if an alternative.

[the Expressions page]:

> Handlebars also supports a deprecated `/` syntax, so you could write the above template as:
>
> ```handlebars
> {{person/firstname}} {{person/lastname}}
> ```

Deprecation of this syntax isn't clearly annotated and can even be seen as a suggestion for an alternative syntax.