Review and update documentation for stochastic sources

!46 (merged) removed the option to pass a scalar instead of a random generator. However, some parts of the documentation still refer to this option.

For example, passing a scalar to StochasticPointSource gives the error message

"invalid generator '{generator}', must be a scalar or a callable"

which is quite confusing. In addition, the official documentation at

https://lisa-simulation.pages.in2p3.fr/gw-response/master/background.html#stochastic-generators

also still refers to the option to pass a scalar.

We should carefully review the documentation and update where appropriate to describe the current behaviour.