Added disambiguation on using raw in expressions

I recently got in trouble using the ``raw`` filter in a ternary, leading to a value being escaped and me not expecting it. The code was : 

```
{{ foo|striptags|length > 250 ? foo|striptags|slice(0, 250) ~ '...' : foo|raw }}
```

When foo's length was under 250 characters, the result of this expression was escaped. Here is the updated documentation to explain why.