Support the \\+ escape which Emacs uses to inhibit forming a button around `xxx'

What the subject says.  Right now, `helpful` incorrectly renders docstrings.

Here it is for the variable `minibuffer-visible-completions`:

<img width="707" height="111" alt="Image" src="https://github.com/user-attachments/assets/cfcf9120-e04b-4f6f-a799-65fc3db34deb" />