correctly document `__call__` for operators

Also improve the documentation for operators in general.

special methods are not included by autodocs by default, and
as class documentation pages are generated by autosummary,
we cannot just explicitly include __call__. We get around this
by using a custom autosummary template for the operator classes.

For cython extension modules, the method signature for __call__
is still not embedded properly.
See https://github.com/cython/cython/issues/1577.