remote.txt: update documentation for 'update' command

Commit b344e1614b (git remote update: Fallback to remote if group does
not exist - 2009-04-06) lets "git remote update" accept individual
remotes as well. Previously this command only accepted remote
groups. The commit updates the command syntax but not the actual
document of this subcommand. Update it.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -182,8 +182,8 @@ actually prune them.
 
 'update'::
 
-Fetch updates for a named set of remotes in the repository as defined by
-remotes.<group>.  If a named group is not specified on the command line,
+Fetch updates for remotes or remote groups in the repository as defined by
+remotes.<group>.  If no group or remote is not specified on the command line,
 the configuration parameter remotes.default will be used; if
 remotes.default is not defined, all remotes which do not have the
 configuration parameter remote.<name>.skipDefaultUpdate set to true will