Document synaptic plasticity support for precise spike times

[clinssen]

It was discovered in #1840 that precise spike timing is not taken into account in synaptic plasticity rules. When two neurons are connected by a plastic synapse, the synapse will faithfully communicate the spike at the precise time to the postsynaptic partner, but the weight update rule will ignore the precise spike time offset, aliasing time to resolution timestep.

The most general solution would be to include the precise spike time when calling nest::Time.get_ms(), which presently ignores this offset and returns only values that are a multiple of the simulation resolution. Although taking the precise timing into account requires just one extra subtraction operation, this raises questions about effects on runtime performance.

We could add this subtraction operation, but enclose it in #ifdef statements coupled to a CMake build flag such as ENABLE_PRECISE_SPIKE_TIMES.

In any case, this issue should be documented. Should it be documented in each affected plasticity model individually? Or only on https://nest-simulator.readthedocs.io/en/v3.1/guides/simulations_with_precise_spike_times.html ?

[clinssen]

Concept PR: #2035

[clinssen]

Documentation about this was already added to the model documentation in #2137.