Fix traffic sign variability attribute description

[thomassedlmayer]

#### Reference to a related issue in the repository Fixes the issue addressed by #623

Description

This change aligns the attribute description of Traffic Sign Variability with the current description of the respective enum (VARIABILITY_VARIABLE). According to the current enum description, the attribute Variability does not only indicate if a traffic sign is stationary or not, but also potentially signifies dynamic traffic signs that could change their type over time (e.g., signs on dynamic sign bridges).

The change does not break existing functionality but rather fixes an existing bug or lack of clarity in the attribute description of Variability.

Take this checklist as orientation for yourself, if this PR is ready for the Change Control Board:

• [x] My suggestion follows the style and contributors guidelines.
• [x] I have taken care about the message documentation and the fields and enums documentation.
• [x] I have done the DCO signoff.
• [x] My changes generate no errors when passing CI tests.
• [ ] I have successfully implemented and tested my fix/feature locally.
• [x] Appropriate reviewer(s) are assigned.