Add `@rtype` to `Docstring` in `remote.webdriver.find_element*`

I've added a parameter `@rtype` to `Docstring` of methods `find_element` and `find_elements` in order to make autocompletion in `PyCharm` for all of `find_element_*` methods of `webdriver`.

![Autocompleting](http://i.imgur.com/K13ran1.png)

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