Helper text to default assignee

[galethil]

• [x] The feature was discussed in discussions of this repository.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Description 💡

Functionality of default assignee in subproject creation is not straight forward. When default assignee is selected it cannot be changed later and forces all workflow item to have selected assignee. Add helper text to explain this behavior.