runtime-config: Require hooks to execute in the host environment

On IRC on 2015-09-28:

  11:28 < jojy_mesos> i guess i was expecting an additional attribute
    for hooks that specifies whether the hooks runs in the host
    namespace or in the container's namespace
  11:28 < lk4d4> they all run in host namespaces

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime-config.md

@@ -12,6 +12,7 @@ Presently there are `Prestart`, `Poststart` and `Poststop`.
 Hooks allow one to run code before/after various lifecycle events of the container.
 Hooks MUST be called in the listed order.
 The state of the container is passed to the hooks over stdin, so the hooks could get the information they need to do their work.
+All hooks execute in the host environment (e.g. the same namespace, cgroups, etc. that apply to the host process).
 
 Hook paths are absolute and are executed from the host's filesystem.