Documentation/git-remote.txt: stress that set-url is not for triangular

Documentation/git-remote.txt: stress that set-url is not for triangular

It seems to be a common mistake to try using a single remote
(e.g. 'origin') to fetch from one place (i.e. upstream) while
pushing to another (i.e. your publishing point).

That will never work satisfactorily, and it is easy to understand
why if you think about what refs/remotes/origin/* would mean in such
a world.  It fundamentally cannot reflect the reality.  If it
follows the state of your upstream, it cannot match what you have
published, and vice versa.

It may be that misinformation is spread by some people.  Let's
counter them by adding a few words to our documentation.

 - The description was referring to <oldurl> and <newurl>, but never
   mentioned <name> argument you give from the command line.  By
   mentioning "remote <name>", stress the fact that it is configuring
   a single remote.

 - Add a reminder that explicitly states that this is about a single
   remote, which the triangular workflow is not about.

Signed-off-by: Junio C Hamano <gitster@pobox.com>