Update the CssSelector component documentation
feature #3909 Update the CssSelector component documentation (stof)

This PR was merged into the 2.3 branch.

Discussion
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Update the CssSelector component documentation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.3+)
| Fixed tickets | n/a

This updates the CssSelector component for changes done in Symfony 2.3. The rewritten component supports more pseudo-classes

Commits
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7db0c58 Update the CssSelector component documentation