clojure-mode.el (clojure-docstring-fill-prefix): New option

* clojure-mode.el (clojure-docstring-fill-prefix): New option.

See https://github.com/clojure-emacs/clojure-mode/issues/241 for a discussion about this.
It's very hard to understand what this does.  We should talk about the effect of the variable, and not just extracting a magic value from the implementation and giving it a name.  

The vim plugin called this `align-multiline-strings`, and that is much better, I think, because it gives some clue as to what the effect of the variable is.