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 more precise in the things that I remember: You can safely use double quotes when the string contains `:`, `{`, `}`, `[`, `]`, `,`, `#`, `-`, `%` and `@`.