Updated ISBN validator
feature #3724 Updated ISBN validator docs (sprain)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #3724).

Discussion
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Updated ISBN validator docs

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no - symfony/symfony/pull/10546
| Applies to    | 2.5

Updated the docs according to the changes in the code.

Commits
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6d64a56 Updated ISBN validator