For C++-relevant stuff, there is the action "[developer] Write C++ API functions header" which creates a header-file with a lot of information in it.
And the main.js in the reaper_www_root-folder of Reaper contains a lot of information about the WebRC.
And if you need to know a lot of more stuff never mentioned in the official docs, see my little collection of technical-docs I made:
https://forum.cockos.com/showthread.php?t=207635
This contains more script-relevant things, including my own approach in reworking the API-docs with more information about parameters and returnvalues.