Fix docs for new org mode

* Rakefile: Comment to remind me how to run it.
* doc/snippet-expansion.org:
* doc/snippet-menu.org: It seems org 9.x got a bit stricter about link
formatting.  Cross-file links *must* use the 'file:' prefix.
* doc/yas-doc-helper.el (yas--document-symbol): Org 9.x also got a bit
more strict about empty lines between a node and its property list.
Make sure not to put a blank line between variable nodes and the
property list.

Rakefile

@@ -50,6 +50,10 @@ task :release => [:package, 'doc:archive'] do
   raise "Not implemented for github yet!"
 end
 
+# rake doc[../htmlize]
+#
+# To do this interactively, load doc/yas-doc-helper, open one of the
+# org files, and do `C-c C-e P'.
 desc "Generate document"
 task :doc, [:htmlize] do |t, args|
   load_path = '-L .'