add nargs docstring

If you want to set the number of arguments for a generic function
you have to use _nargs.

f._nargs = (1,) means that f must take only 1 argument

This attribute should be set before any application of f is made.
For example, if f(x,y) is initialized before f._nargs is set to
(1,) this will create a cached version with two args without raising
an error since nargs was None at the time of instantiation. Any *other*
2-arg calls will raise an error about an arg count mismatch, however.