Fix the definition of customizing form's global errors.
Added note about customizing errors for different field types.
Minor rewording. Added tip about meaning of `compound` var and expanded
the example code to contain `else` statements.
Applied some changes according to comments.
bug #3543 Fix the definition of customizing form's global errors. (mtrojanowski)

This PR was merged into the 2.3 branch.

Discussion
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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

Commits
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6f6fcca Applied some changes according to comments.
e3b20d8 Minor rewording. Added tip about meaning of `compound` var and expanded the example code to contain `else` statements.
1e278c4 Added note about customizing errors for different field types.
236d06b Fix the definition of customizing form's global errors.