Add note to `aeif` models about refractory period and peculiarities

[jessica-mitchell]

Fixes #3088

This PR adds a note to all aeif models, including the reference if needed based on the comment in #3088. The technical reason was left out, since the focus should be on usage imo.

See output from Read the Docs here: https://nest-simulator--3296.org.readthedocs.build/en/3296/models/aeif_psc_exp.html