shell doc: emphasize purpose and security model

The original git-shell(1) manpage emphasized that the shell supports
only git transport commands.  As the shell gained features, that
emphasis and focus in the manual has been lost.  Bring it back by
splitting the manpage into a few short sections and fleshing out each:

 - SYNOPSIS, describing how the shell gets used in practice
 - DESCRIPTION, which gives an overview of the purpose and guarantees
   provided by this restricted shell
 - COMMANDS, listing supported commands and restrictions on the
   arguments they accept
 - INTERACTIVE USE, describing the interactive mode

Also add a "see also" section with related reading.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shell.txt

@@ -9,25 +9,61 @@ git-shell - Restricted login shell for Git-only SSH access
 SYNOPSIS
 --------
 [verse]
-'git shell' [-c <command> <argument>]
+'chsh' -s $(command -v git-shell) <user>
+'git clone' <user>`@localhost:/path/to/repo.git`
+'ssh' <user>`@localhost`
 
 DESCRIPTION
 -----------
 
-A login shell for SSH accounts to provide restricted Git access. When
-'-c' is given, the program executes <command> non-interactively;
-<command> can be one of 'git receive-pack', 'git upload-pack', 'git
-upload-archive', 'cvs server', or a command in COMMAND_DIR. The shell
-is started in interactive mode when no arguments are given; in this
-case, COMMAND_DIR must exist, and any of the executables in it can be
-invoked.
+This is a login shell for SSH accounts to provide restricted Git access.
+It permits execution only of server-side Git commands implementing the
+pull/push functionality, plus custom commands present in a subdirectory
+named `git-shell-commands` in the user's home directory.
 
-'cvs server' is a special command which executes git-cvsserver.
+COMMANDS
+--------
+
+'git shell' accepts the following commands after the '-c' option:
+
+'git receive-pack <argument>'::
+'git upload-pack <argument>'::
+'git upload-archive <argument>'::
+	Call the corresponding server-side command to support
+	the client's 'git push', 'git fetch', or 'git archive --remote'
+	request.
+'cvs server'::
+	Imitate a CVS server.  See linkgit:git-cvsserver[1].
+
+If a `~/git-shell-commands` directory is present, 'git shell' will
+also handle other, custom commands by running
+"`git-shell-commands/<command> <arguments>`" from the user's home
+directory.
+
+INTERACTIVE USE
+---------------
 
-COMMAND_DIR is the path "$HOME/git-shell-commands". The user must have
-read and execute permissions to the directory in order to execute the
-programs in it. The programs are executed with a cwd of $HOME, and
-<argument> is parsed as a command-line string.
+By default, the commands above can be executed only with the '-c'
+option; the shell is not interactive.
+
+If a `~/git-shell-commands` directory is present, 'git shell'
+can also be run interactively (with no arguments).  If a `help`
+command is present in the `git-shell-commands` directory, it is
+run to provide the user with an overview of allowed actions.  Then a
+"git> " prompt is presented at which one can enter any of the
+commands from the `git-shell-commands` directory, or `exit` to close
+the connection.
+
+Generally this mode is used as an administrative interface to allow
+users to list repositories they have access to, create, delete, or
+rename repositories, or change repository descriptions and
+permissions.
+
+SEE ALSO
+--------
+ssh(1),
+linkgit:git-daemon[1],
+contrib/git-shell-commands/README
 
 GIT
 ---