describe how to access form errors
[Form] document $deep and $flatten of getErrors()
feature #4169 [Components][Form] document $deep and $flatten of getErrors() (xabbuh)

This PR was merged into the 2.5 branch.

Discussion
----------

[Components][Form] document $deep and $flatten of getErrors()

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#9918)
| Applies to    | 2.5+
| Fixed tickets | #3660

This is based on #4168.

Commits
-------

0245e91 [Form] document $deep and $flatten of getErrors()
4221db8 describe how to access form errors