There’s two parts to a Nyquist plug-in. There’s the Nyquist code, and the plug-in headers.
The headers are described here (plus links on that page): https://wiki.audacityteam.org/wiki/Nyquist_Plug-ins_Reference
There’s a list of documentation here: https://forum.audacityteam.org/t/manuals-and-reference-material/32817/1