doc: remote: say that it only affects the local repository

Nothing that "git remote" does changes a remote repository.  "set-head"
updates the local refs/remotes/<name>/HEAD, not the remote's own HEAD;
"prune" deletes stale remote-tracking branches, not branches on the
remote; and so on.  The manual page never says so, and wording such as
"Set or delete the default branch ... for the named remote" can be read
as acting on the remote itself.  It was recently misread that way in a
discussion on another project's mailing list.

Say it once, near the top of the DESCRIPTION, rather than in the
description of each subcommand.

Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.adoc

@@ -28,6 +28,11 @@ DESCRIPTION
 
 Manage the set of repositories ("remotes") whose branches you track.
 
+`git remote` changes only the local repository, i.e. its configuration
+and its refs, and never modifies a remote repository.  Some subcommands,
+such as `show`, `prune`, `update` and `set-head --auto`, contact a
+remote repository to read from it.
+
 
 OPTIONS
 -------