ImageStyle::buildUrl() should generate root-relative URLs instead of absolute

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

As a bug will need tests before review.

As the original reporter of the issue, I think it's time to close this. Frankly the title contradicts itself, a root-relative URL isn't a URL. As Berdir pointed out, there have been related issues reported, some merged, which I'm now linking to this issue. Although we could convert this to a simple documentation update (my proposed alternate approach, patch attached for posterity), one might argue that the buildUrl() docs have always linked to the relevant methods for converting to root-relative links, so even this documentation patch is not really needed. Thanks to everyone for their contributions, but let's help clean up the issue queue by closing this.