Update `Reserved words` section in `Variable names` article with `rostertitle`

https://docs.mysurvey.solutions/questionnaire-designer/components/variable-names/

should mention `rostertitle` as a reserved word as per:

<img width="949" height="310" alt="Image" src="https://github.com/user-attachments/assets/960bf2a3-43ed-4ef6-8519-b2aa09d2ca8b" />