help: document the `path://` url scheme
If we want people to use it, we need to document it.
Differential Revision: https://phab.mercurial-scm.org/D10372
help: point to `hg help urls` in `hg help config.paths`
This seems useful to point at what people can put as value for these config.
Differential Revision: https://phab.mercurial-scm.org/D10371