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.
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