Update file_uploads.rst

Code examples do not apply to Doctrine EventSubscribers scenario, on preUpdate.
Cheers.
Update file_uploads.rst

Updated as per @RyanWeaver's suggestions.
Rewording Caution paragraph at line 413
Going back to the ``Caution`` alternative.
feature #3525 Update file_uploads.rst (juanmf)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3525).

Discussion
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Update file_uploads.rst

Code examples do not apply to Doctrine EventSubscribers scenario, on preUpdate.
Cheers.

Commits
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075612a Going back to the ``Caution`` alternative.
2e21a0b Rewording Caution paragraph at line 413
3b0c75d Update file_uploads.rst
25539c5 Update file_uploads.rst