shell: Rewrite documentation and improve error message

Update the documentation of 'git shell' to mention the interactive
mode and COMMAND_DIR. Also provide a hint when interactive mode is not
available in the shell.
Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Reviewed-by: NGreg Brockman <gdb@MIT.EDU>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-shell.txt

@@ -3,24 +3,30 @@ git-shell(1)
 
 NAME
 ----
-git-shell - Restricted login shell for GIT-only SSH access
+git-shell - Restricted login shell for Git-only SSH access
 
 
 SYNOPSIS
 --------
-'$(git --exec-path)/git-shell' -c <command> <argument>
+'git shell' [-c <command> <argument>]
 
 DESCRIPTION
 -----------
-This is meant to be used as a login shell for SSH accounts you want
-to restrict to GIT pull/push access only. It permits execution only
-of server-side GIT commands implementing the pull/push functionality.
-The commands can be executed only by the '-c' option; the shell is not
-interactive.
-
-Currently, only four commands are permitted to be called, 'git-receive-pack'
-'git-upload-pack' and 'git-upload-archive' with a single required argument, or
-'cvs server' (to invoke 'git-cvsserver').
+
+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.
+
+'cvs server' is a special command which executes git-cvsserver.
+
+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.
 
 Author
 ------

shell.c

@@ -149,8 +149,11 @@ int main(int argc, char **argv)
 	} else if (argc == 1) {
 		/* Allow the user to run an interactive shell */
 		cd_to_homedir();
-		if (access(COMMAND_DIR, R_OK | X_OK) == -1)
-			die("Sorry, the interactive git-shell is not enabled");
+		if (access(COMMAND_DIR, R_OK | X_OK) == -1) {
+			die("Interactive git shell is not enabled.\n"
+			    "hint: ~/" COMMAND_DIR " should exist "
+			    "and have read and execute access.");
+		}
 		run_shell();
 		exit(0);
 	} else if (argc != 3 || strcmp(argv[1], "-c")) {