Fix the definition of customizing form's global errors.

The documentation for customizing global errors was actually still referring to the way it was done in 2.0. (where two different blocks were used: `field_errors` and `form_errors`). Since 2.1. only `form_errors` is used and errors are differentiated using the `compound` variable (set to `true` for form errors).

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3542