Update synapse keywords used in model filter docs

[jessica-mitchell]

Based on review with @ddahmen and initial view at hackathon with other developers,
here is the suggested list of synaspe keywords to use to filter models

• chemical - Unidirectional spike transmission from presynaptic to postsynaptic neuron
• electrical - Bidirectional voltage-based transmission
• abstract - Non-biological models, often used for rate-based simulations
• rate - Rate-coded transmission (continuous signals)
• learning - Learning signal connections for e-prop
• functional - Synapses with dynamic functional properties
• static - Static synapses with no plasticity
• stochastic - stochastic spike transmission where neurotransmitter release is probabliistic
• stp - Short-term plasticity
• stdp - Spike-timing dependent plasticity
• 3-factor - 3-factor plasticity rules (e.g., clopath, urbanczik, vogels_sprekeler)
• astrocyte - Astrocyte coupling models

Structural would be another, but we don't have any models that support this specifically

Decision needed

this list removes terms like Hill-Tononi and Clopath from the synapse models. This means that you cannot filter for those terms and find the neuron and synapse models that correspond to each other- we can re-add these back of course if that is desired.