Documented the characters that provoke a YAML escaping string

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4197
To be honest, I have some problems understanding the code of the component. Sometimes they escape with single quotes (https://github.com/symfony/symfony/blob/f940d92a32e4d70cbe045ab8e1b3c70d3eb6061e/src/Symfony/Component/Yaml/Escaper.php#L66) and others with double quotes (https://github.com/symfony/symfony/blob/f940d92a32e4d70cbe045ab8e1b3c70d3eb6061e/src/Symfony/Component/Yaml/Escaper.php#L42).