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 like this table (and sphinx doesn't like it that there is an empty cell). It's a list, not a table imo. So either use an inline list: "`:`, `{`, `}`, `[`, `]`, `,`, ... and `\\`"