Name of the main help file. All distributed help files should be placed together in one directory. Additionally, all "doc" directories in 'runtimepath' will be used. Environment variables are expanded |:set_env|. For example: "$VIMRUNTIME/doc/help.txt". If $VIMRUNTIME is not set, $VIM is also tried. Also see |$VIMRUNTIME| and |option-backslash| about including spaces and backslashes. This option cannot be set from a |modeline| or in the |sandbox|, for security reasons.