Documented the characters that provoke a YAML escaping string

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4197
What about?

> Quoted styles are useful when a string starts or end with one or more relevant spaces, because unquoted strings are trimmed on both end when parsing their contents. Quotes are required when the string contains special or reserved characters.