Recapping the situations that need test cases before this can be merged:

* behaviour when "help" is entered at the interactive help prompt (as the current behaviour is correct and should *not* change)
* behaviour when calling "help(help)"
* behaviour when calling "help('help')"
* behaviour when running "python3 -m pydoc help"

In the latter 3 cases, the default site._Helper docs currently shown are not particularly helpful.

It does make me wonder whether it might be worth defining a __help__ magic method that completely overrides what help(obj) displays. It would limit the fix to 3.7+, but it would make it possible to do this cleanly just by defining __help__ on site._Helper.