Onion-Location html meta tag instructions should include url (or say that it doesn't)

On https://community.torproject.org/onion-services/advanced/onion-location/ there is a section "Using an HTML attribute" which tells you how to add a piece to your html, rather than needing to edit your webserver config to add a new http header.

But these instructions as currently written will send users to your onion service root ("/") rather than to the equivalent (relative) page on your onion site.

We should either (1) document how to make it send people to the right page on the onion site, or if that's hard (e.g. because html itself can't do it, and you'd need to construct the html out of whatever is building your site content, which will vary wildly from site to site), we should (2) document this limitation and encourage people to do the http header when possible.