aeif-model documentation does not point out peculiarities

[heplesser]

The current documentation for the aeif models does not point out two peculiarities of these models:

1. The default refractory period to these models is zero. This is consistent with the model definition in Brette & Gerstner, but most likely unexpected, since NEST models generally have a non-zero refractory period.
2. Because NEST uses an adaptive stepsize solver for the aeif ODEs with spike detection inside the update loop, aeif neurons can fire multiple spikes in a single time step if the refractory period is zero (the default). This can lead to exploding spike numbers and extreme slow-down of simulations.

Documentation should point this out clearly in note boxes and recommend users to set t_ref > 0.

[github-actions[bot]]

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