httppeer: consolidate _requestbuilder assignments and document I collapsed multiple assignments because they don't appear to be necessary. We don't invoke the requestbuilder in anything called during __init__. So there's no reason to not define it earlier AFAICT. This seemingly useless attribute is actually an extension point. Document it as such. (A previous version of this patch removed the attribute because it appeared to just be an alias.) Differential Revision: https://phab.mercurial-scm.org/D2723