Adding/removing methods to a multimethod should update its docstring to list known implementations

[camsaul]

Something like this would be a game-changer. See also #46

(m/defmulti my-multimethod
  "WOW"
  keyword)

(:doc (meta #'my-multimethod))
-> 
"WOW"

(m/defmethod my-multimethod :x
  [_])

(:doc (meta #'my-multimethod))
->
"WOW

Primary Method Implementations:

* `:x`