page_url docs: explain handling of incorrect reverse IDs

In previous projects I have refused to use the ``{% page_url %}`` template tag because I was worried about 500 errors in production if an editor changed a page's reverse ID in the admin. It wasn't until I looked through the source code that I discovered an exception is only raised in debug mode, not production, and also it will optionally email people to inform them of a broken link, which is really useful!

However the docs don't mention this at all. If they had done, I would have used the tag a lot sooner. I think this information should be in the docs, so I have added it in this pull request.