tools: document -d/--daemonize for lxc-execute

Closes #2280.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

doc/lxc-execute.sgml.in

@@ -50,6 +50,7 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
     <cmdsynopsis>
       <command>lxc-execute</command>
       <arg choice="req">-n <replaceable>name</replaceable></arg>
+      <arg choice="opt">-d</arg>
       <arg choice="opt">-f <replaceable>config_file</replaceable></arg>
       <arg choice="opt">-s KEY=VAL</arg>
       <arg choice="opt">-- <replaceable>command</replaceable></arg>
@@ -124,6 +125,20 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 	  </para>
 	</listitem>
       </varlistentry>
+
+      <varlistentry>
+	<term>
+	  <option>-d, --daemon</option>
+	</term>
+	<listitem>
+	  <para>
+	    Run the container as a daemon. As the container has no
+	    more tty, if an error occurs nothing will be displayed,
+	    the log file can be used to check the error.
+	  </para>
+	</listitem>
+      </varlistentry>
+
       <varlistentry>
 	<term><option>--</option></term>
 	<listitem>

src/lxc/tools/lxc_execute.c

@@ -104,6 +104,7 @@ and execs COMMAND into this container.\n\
 \n\
 Options :\n\
   -n, --name=NAME      NAME of the container\n\
+  -d, --daemon         Daemonize the container\n\
   -f, --rcfile=FILE    Load configuration file FILE\n\
   -s, --define KEY=VAL Assign VAL to configuration variable KEY\n\
   -u, --uid=UID        Execute COMMAND with UID inside the container\n\