DOC: document default argument values for some arpack functions

closes https://github.com/scipy/scipy/issues/3829
I've seen both ways, so I was just guessing.  Does `integer` have any meaning?  For example hypothetically it might be more flexible allowing `int` and `np.float16` and whatever else acts like an integer, whereas `int` might hypothetically mean exactly Python `int` type.  If not then I'll change them all to `int`.