Refinements and additions to routing method args

[crandmck]

From #701:

I think the description of the path & callback arguments is also applicable to router.all, router.METHOD, and router.use, but I wasn't 100% sure.

Since the method signatures (in the headings, e.g. http://expressjs.com/en/4x/api.html#app.all) have callback [, callback ...] , I wonder if we need to specify "A series of middleware functions (separated by commas)." since that's implied by the signature? Maybe it's clear either way? Or perhaps we should just have callback(s) in the signature?