nil. From the docstring (emphasis mine):
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.
Please don't do this, though. There is a reason for this setting: Emacs shows docstrings exactly like the appear in your code. If you fill your docstrings at 80 characters, you force your readers to read them filled at 80 characters. Even those readers, who view docstrings on their 11 inch laptops in horizontally split buffers. And they have no chance to change the filling in any way.