Add warning about SYS_BOOT capability with pre-3.4 kernels and pre-0.8 LXC.

docs/sources/installation/kernel.rst

@@ -25,6 +25,7 @@ If you cannot or do not want to use the "official" kernels,
 here is some technical background about the features (both optional and
 mandatory) that docker needs to run successfully.
 
+
 Linux version 3.8 or above
 --------------------------
 
@@ -39,6 +40,15 @@ The symptoms include:
 - kernel crash causing the machine to freeze for a few minutes, or even
   completely.
 
+Additionally, kernels prior 3.4 did not implement ``reboot_pid_ns``,
+which means that the ``reboot()`` syscall could reboot the host machine,
+instead of terminating the container. To work around that problem,
+LXC userland tools (since version 0.8) automatically drop the ``SYS_BOOT``
+capability when necessary. Still, if you run a pre-3.4 kernel with pre-0.8
+LXC tools, be aware that containers can reboot the whole host! This is
+not something that Docker wants to address in the short term, since you
+shouldn't use kernels prior 3.8 with Docker anyway.
+
 While it is still possible to use older kernels for development, it is
 really not advised to do so.