Document single-character regular expressions not being supported
Document single-character regular expressions not being supported, and follow previous examples style.
I lost a considerable amount of time realizing GitLab does not support single-character regular expressions. Having the example follow the style of previous examples would have been helpful in understanding part of what I was trying was not the problem.
This MR fixes the same issue pointed out in !107699 (closed), and is opened in the community fork to avoid available CI minutes issues.
Edited by Andrés Delfino