Remove not existing "relative ordering" feature from docs.

[WloHu]

Title. Doc page is here: http://pytest-ordering.readthedocs.org/ .

If it's a "someday" feature then explicitly state that in the doc!
Just imagine how f*****g mad your lies made me! At least source code doesn't lie!

[ftobia]

I'm sorry to have upset you. In my defense, the note at the top of the page tries to warn of this: "Notably, some of this documentation may be aspirational in nature." I welcome pull requests to improve the documentation.