docs: add warning about 4x backslash escaping for regex fields

  only the **first** matched response is sent back.

    - **param**: _string_ - A parameter that is matched **exactly** to the input of trainees.

      Can be written as [Python regular expression](https://docs.python.org/3/library/re.html) if parameter `regex` is set to true.

      **Note**: if you need to match a literal backslash (`\`) in trainee input,

      you must write **4× as many backslashes** in the definition.

      Example: to match `\` write `\\\\` in `param`; to match `\\` write `\\\\\\\\`.

    - **regex**: _bool, default=False_ - Determines whether parameter `param` is matched as a string or a regex.

    - **time**: _int, default=0_ - Time since the beginning of the exercise in minutes after which this response can be matched.

      If the trainees use correct param but this time was not reached yet, it will not match.

- **correct_answer**: _string_ - the correct answer for this question

- **regex**: _bool, default=False_ - determines whether `correct_answer` is used

  as a regex during answer evaluation

  **Note**: if you need to match a literal backslash (`\`) in the answer,

  you must write **4× as many backslashes** in the definition.

  Example: to match `\` write `\\\\` in `correct_answer`; to match `\\` write `\\\\\\\\`.

- **correct**: _control, default=empty_ - milestone modifications triggered when the given answer is correct,

  `milestone_condition` and `roles` cannot be specified

- **incorrect**: _control, default=empty_ - milestone modifications triggered when the given answer is incorrect,