Add missing ipc_mode documentation

Author: shin-

docs/hostconfig.md

@@ -86,6 +86,7 @@ for example:
 * read_only (bool): mount the container's root filesystem as read only
 * pid_mode (str): if set to "host", use the host PID namespace inside the
   container
+* ipc_mode (str): Set the IPC mode for the container
 * security_opt (list): A list of string values to customize labels for MLS
   systems, such as SELinux.
 * ulimits (list): A list of dicts or `docker.utils.Ulimit` objects. A list