enclose YAML strings containing % with quotes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4029

Thanks @iamdto, did you spot some other occurences in the docs?
to be exact, YAML 1.1 has an issue only when ``%`` starts the values, so this change is not necessary