added a note about types we support
minor #3802 Add a note about which types can be used in Symfony (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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Add a note about which types can be used in Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

see symfony/symfony#10701

Commits
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bf9d509 added a note about types we support