Update UPGRADE-2.3.md to account for #9388
minor #9804 [Form] Update UPGRADE-2.3.md to account for #9388 (ureimers)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #9804).

Discussion
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[Form] Update UPGRADE-2.3.md to account for #9388

Added documentation for how to correctly pre-fill a form using the form's `data` option.
The "old" and also wrong way of doing it broke with Symfony 2.3.7 (with https://github.com/symfony/symfony/pull/9388 to be precise) and this short documentation should help others to fix the problem and do it right.

Commits
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5e06535 [Form] Update UPGRADE-2.3.md to account for #9388