Make XPath on document addition more helpful

[madoleary]

We should explain why we're asking for xpath, and we should also give a couple of examples with their own succinct explanations of format.

[arthurlutz]

Would recommending and extension to help find the Xpath such as https://addons.mozilla.org/en-US/firefox/addon/xpath_finder/ be helpful ?

So far, am a bit disappointed with the said extension since it find xpath with indices for elements which might break when the page changes.