Less code mess in commands:help_cmd()
Add comments to a couple of cfg/config.h defines
Fix `:help` (no arguments) when 'vimhelp' is on

And automatically installed Vim documentation is used.

Extract constant.

Thanks to ranousse.
Merge branch 'fix-no-args-vimdoc-help'

Fix `:help` (no arguments) when 'vimhelp' is on and automatically
installed Vim documentation is used.

Thanks to ranousse.

Addresses #59, but there also will be follow refactoring.