Fully rework docs for group-accessible repositories

Just tell the user what to do instead of explaining too many details.
I've dropped the read-only variant as it actually has no representation
in the local and sftp backends. Instead it relied on both backends
initially creating all directories, which can't actually be guaranteed.

Based on a suggestion by @brad2014 in significant parts.

doc/030_preparing_a_new_repo.rst

@@ -786,56 +786,45 @@ On MSYS2, you can install ``winpty`` as follows:
 Group accessible repositories
 *****************************
 
-Since restic version 0.14 local and SFTP repositories can be made
-accessible to members of a system group. To control this we have to change
-the group permissions of the top-level ``config`` file and restic will use
-this as a hint to determine what permissions to apply to newly created
-files. By default ``restic init`` sets repositories up to be group
-inaccessible.
+Since restic version 0.14, both local and SFTP repositories can be made
+accessible to all the members of a given UNIX group on the repository host.
 
-In order to give group members read-only access we simply add the read
-permission bit to all repository files with ``chmod``:
-
-.. code-block:: console
-
-    $ find /srv/restic-repo -type f -exec chmod 440 '{}' \;
-    $ find /srv/restic-repo -type d -exec chmod 750 '{}' \;
-
-This serves two purposes: 1) it sets the read permission bit on the
-repository config file triggering restic's logic to create new files as
-group accessible and 2) it actually allows the group read access to the
-files and directories.
-
-.. note:: By default files on Unix systems are created with a user's
-          primary group as defined by the gid (group id) field in
-          ``/etc/passwd``. See `passwd(5)
-          <https://manpages.debian.org/latest/passwd/passwd.5.en.html>`_.
-
-For read-write access things are a bit more complicated. When users other
-than the repository creator add new files in the repository they will be
-group-owned by this user's primary group by default, not that of the
-original repository owner, meaning the original creator wouldn't have
-access to these files. That's hardly what you'd want.
-
-To make this work we can employ the help of the ``setgid`` permission bit
-available on Linux and most other Unix systems. This permission bit makes
-newly created directories or files inherit both the group owner (gid) and
-setgid bit (only for directories) from the parent directory. Setting this
-bit requires root but since it propagates down to any new directories we
-only have to do this privileged setup once:
+To permit multiple users to use a repository, first run ``restic init`` to
+create it, if necessary. Then, some manual intervention is currently required.
+Run the following commands over the repository files themselves, which give
+the required permissions (and hints to restic). Thereafter, restic commands
+can be run against that repository by any member of a given UNIX group.
+
+To allow UNIX group ``restic-users`` to read and write to a repository at
+``/srv/restic-repo``, run the following commands:
 
 .. code-block:: console
 
+    $ chgrp -R restic-users /srv/restic-repo
     $ find /srv/restic-repo -type f -exec chmod 440 '{}' \;
     $ find /srv/restic-repo -type d -exec chmod 2770 '{}' \;
 
-This sets the ``setgid`` bit on all existing directories in the repository
-and then grants read/write permissions for group access.
+(Internally, the group read permission on the ``config`` file tells restic to
+create all future files and directories inside the repository with
+group-read permission, and the ``setgid`` mode bit on directories causes
+restic to set the group of each newly created file to the group of its parent
+directory. They thus remain accessible to all members of group ``restic-users``,
+regardless of which user created them.)
 
 .. note:: To manage who has access to the repository you can use
-          ``usermod`` on Linux systems, to change which group controls
-          repository access ``chgrp -R`` is your friend.
+          ``usermod`` on Linux systems.
 
+For a repository accessed via SFTP, note that the user used for the SFTP connection
+should belong to the appropriate group.
+
+.. code-block:: console
+
+    $ restic backup -r sftp:restic@repohost:/srv/restic-repo
+
+In the example, the command could be run by the local user ``root`` who can read
+all the files on the client host, and send them for backup using a remote user ``restic``
+to add them to the repository. In this example, ``restic`` should be part of the
+``restic-users`` group on ``repohost``.
 
 Repositories with empty password
 ********************************