emacs-lisp-docstring-fill-column

emacs-lisp-docstring-fill-column is a variable defined in `lisp-mode.el'.
Its value is

65

• This variable is safe as a file local variable if its value
  satisfies the predicate which is a byte-compiled expression.
  

Documentation:
Value of `fill-column' to use when filling a docstring.
Any non-integer value means do not use a different value of
`fill-column' when filling docstrings.

You can customize this variable.