Documented the characters that provoke a YAML escaping string

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4197
I don't understand your comments. When you say: *"... and also if it contains special chars"* ... are you referring to "quoted styles are useful in that case too" or "they are trimmed on both ends in that case too". I think that the resulting phrase would be too long and complex. Could we reword it in two or three short sentences? Thanks!