Consider a mechanism to annotate a temp spec that rewrites part of another one

[tidoust]

We have a mechanism to flag forks of a spec. We also have a mechanism to flag specs that are written as delta specs over their previous level. Now, once in a while, a spec rewrites part of another one without being directly related to it. HTML Ruby Extensions is the main example here. Having some flag in browser-specs would allow us to trap and avoid conflicts in Webref extracts automatically without having to go through a patch.

Via #1477 (comment)