Allow fontifying helm-org-headings.

* helm-org.el (helm-org-headings-fontify): New user var.
(helm-get-org-candidates-in-file,
helm-org-get-candidates): Use it.
Add a new command for all sources documentation.

* helm-help.el (helm-documentation): New command.
(helm-documentation-file): New user var.
(helm-help--string-list): new internal.