FrontEndTokenExecute["Save"] saves the current notebook, but
FrontEndTokenExecute["Wolfram Documentation"] does not open the documentation notebook. Why?
Working from kglr's and b3m2a1's comments, I will record a CW answer.
The menu tokens are stored in the file
MenuSetup.tr buried deep within the Mathematica installation. To look at it you can execute the following:
SystemOpen[ FrontEndExecute @ FrontEnd`FindFileOnPath["MenuSetup.tr", "PrivatePathsTextResources"]]
The result will be the desired file appearing in new front-end window. You can then search in it for "Help". which will give you
and thus revealing the token you are looking for.
The above worked for me. I'm running V11.3 on OS X 10.13.4. Alexey Popkov says it didn't work for him. He recommends
FilePrint[ FrontEndExecute @ FrontEnd`FindFileOnPath["MenuSetup.tr", "PrivatePathsTextResources"]]
In this case, the result will be the desired file printed in the current Notebook.
Once again, as in fantasy fiction, knowing something's true name gives power over it.