runc kill: drop -a option

As of previous commit, this is implied in a particular scenario. In
fact, this is the one and only scenario that justifies the use of -a.

Drop the option from the documentation. For backward compatibility, do
recognize it, and retain the feature of ignoring the "container is
stopped" error when set.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

man/runc-kill.8.md

@@ -4,7 +4,7 @@
 **runc-kill** - send a specified signal to container
 
 # SYNOPSIS
-**runc kill** [**--all**|**-a**] _container-id_ [_signal_]
+**runc kill** _container-id_ [_signal_]
 
 # DESCRIPTION
 
@@ -15,15 +15,6 @@ A different signal can be specified either by its name (with or without the
 **SIG** prefix), or its numeric value. Use **kill**(1) with **-l** option
 to list available signals.
 
-# OPTIONS
-**--all**|**-a**
-: Send the signal to all processes inside the container, rather than
-the container's init only. This option is implied when the _signal_ is **KILL**
-and the container does not have its own PID namespace.
-
-: When this option is set, no error is returned if the container is stopped
-or does not exist.
-
 # EXAMPLES
 
 The following will send a **KILL** signal to the init process of the