Add some documentation for the suexec command

Change-Id: If6cb9cf54f5000fd56a728d767ef0404289ce0e4

Documentation/cmd-index.txt

@@ -105,6 +105,9 @@ link:cmd-kill.html[kill]::
 link:cmd-show-queue.html[ps]::
 	Alias for 'gerrit show-queue'.
 
+link:cmd-suexec.html[suexec]::
+	Execute a command as any registered user account.
+
 GERRIT
 ------
 Part of link:index.html[Gerrit Code Review]

Documentation/cmd-suexec.txt

@@ -0,0 +1,50 @@
+suexec
+======
+
+NAME
+----
+suexec - Execute a command as any registered user account
+
+SYNOPSIS
+--------
+[verse]
+'ssh' -p <port> 'Gerrit Code Review'@localhost -i <private host key> 'suexec' \--as <EMAIL> [\--from HOST:PORT] [\--] [COMMAND]
+
+DESCRIPTION
+-----------
+The suexec command can only be invoked by the magic user Gerrit Code Review
+and permits executing any other command as any other registered user account.
+
+OPTIONS
+-------
+
+\--as::
+	Email address of the user you want to impersonate.
+\--from::
+	Hostname and port of the machine you want to impersonate the command
+	coming from.
+COMMAND::
+	Gerrit command you want to run.
+
+ACCESS
+------
+Caller must be the magic user Gerrit Code Review using the SSH daemon's host key
+or a key on this daemon's peer host key ring.
+
+SCRIPTING
+---------
+This command is intended to be used in scripts.
+
+EXAMPLES
+--------
+
+Approve the change with commit c0ff33 as "Verified +1" as user bob@example.com
+=====
+	$ sudo -u gerrit ssh -p 29418 -i site_path/etc/ssh_host_rsa_key \
+	'Gerrit Code Review'@localhost suexec --as bob@example.com -- \
+	gerrit approve --verified=+1 c0ff33
+=====
+
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]