Collection of minor fixes: Typos, formatting, improvements, logic

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | -
| Applies to    | 2.2
| Fixed tickets | -

Please note that some changes only make sense in the context of the appropriate section. For instance, the description of form themes for individual form fields introduces an example with a ``first_name`` and a  ``last_name`` field, but what follows is code with a product's ``name`` field. Similarly, in an Assetic cookbook, the code for the prod environment shows a dev controller etc.