[config-manager] Allow to specify the "main" section

Also change the name of the "crepo" argument from "repo" to "section" and
fix its description to unite it with the description in documentation.

plugins/config_manager.py

@@ -41,8 +41,8 @@ class ConfigManagerCommand(dnf.cli.Command):
     @staticmethod
     def set_argparser(parser):
         parser.add_argument(
-            'crepo', nargs='*', metavar='repo',
-            help=_('repo to modify'))
+            'crepo', nargs='*', metavar='section',
+            help=_('configuration sections to manage, "main" or repoid'))
         parser.add_argument(
             '--save', default=False, action='store_true',
             help=_('save the current options (useful with --setopt)'))
@@ -111,6 +111,8 @@ def modify_repo(self):
         not_matching_repos_id = set()  # IDs of not matched repositories
 
         def match_repos(key, add_matching_repos):
+            if key == 'main':
+                return
             matching = self.base.repos.get_matching(key)
             if not matching:
                 not_matching_repos_id.add(name)