Force multi-line rendering of signatures at the directive level.

[picnixz]

I was documenting the testing module in #11972 when I saw that some of the signatures I have are a bit too long. Instead of enabling #11011 for the whole project, I wanted to have the reverse flag for single-line-parameter-list in which we could specify the line length of a single signature or force multi-line rendering (e.g., :multi-line-parameter-list: and :parameter-list-max-width: 80). I'm opening the issue now but I'll work on it either this w-e or in the upcoming weeks.