Update kill.md

Added Note to show users that signals will not propagate to the container if the preferred exec form isn't used.

Signed-off-by: Eric Rosenberg <ehaydenr@gmail.com>

docs/reference/commandline/kill.md

@@ -19,3 +19,8 @@ parent = "smn_cli"
 
 The main process inside the container will be sent `SIGKILL`, or any
 signal specified with option `--signal`.
+
+> **Note:**
+> `ENTRYPOINT` and `CMD` in the *shell* form run as a subcommand of `/bin/sh -c`,
+> which does not pass signals. This means that the executable is not the container’s PID 1
+> and does not receive Unix signals.