docs: fix mirroring information about git pushurl

I was passing through that docs section yesterday and I notice that adding a `pushurl` does not actually replaces the `url`. Git will leave the `url` there but uses it for fetch solely, we still need to add both `pushurl` to push to both places. I think changing the text would look more correct. I also add a bit of tag lang to the markdown, a URL to git-push official docs, and fix a bit the examples to make them more consistent.