Documented sitemap_resorce.url vs sitemap_resource.path for link_to

@bhollis was kind to [explain](https://github.com/middleman/middleman/issues/1096#issuecomment-29444434) why `link_to sitemap_resource.data.title, sitemap_resource.data.url, relative: true` produces an error.

I documented it so that less people will step on the same rake.

To view the resulting text rendered by Github, open it [here](https://github.com/lolmaus/middleman-guides/blob/c5d24a9ec12cf5e788af34d859bb3e5d09091d9b/source/basics/helpers.html.markdown) and Ctrl+F/Cmd+F for "If the link_to helper fails to determine".

I'm worried that you may find my writing style not matching documentation style. Feel free to adjust!