Document uuid directive

* doc/snippet-development.org: Mention uuid in directive list.
* yasnippet.el (yas-define-snippets): Fix docstring grammar.
Document type directive

* doc/snippet-development.org: Add type to directive list.
Fix publishing with org 7.9

* doc/yas-doc-helper.el: Only use org-html-publish-to-html when it's
  fbound.