update kernel reqs doc; recommend updates on RHEL

We've received numerous reports for issues which have been fixed in newer kernels. We've also received reports from users who were trying to get Docker to run on systems where a Docker compatible kernel isn't available by default.

Issues like #1300, #7211 and #9256 have pointed out the fact that there are users out there who are a) trying to get Docker to run on unsupported systems, b) running kernels which are no longer supported (like kernel 3.8 on Ubuntu 12.04) or c) running supported kernels, but they're not installing system updates.

We need to make sure users are aware that they need to keep systems updated.

This PR also changes the Ubuntu docs to make it clear that 12.04 should actually be used together with kernel 3.13 (lts-trusty).
We need to make sure people understand that 3.8 is the absolute minimum required to start Docker. It's not in any way recommended. However, convincing people that they need a newer kernel is very difficult. This is at least as difficult as convincing them to install system updates so they get updated kernel packages.

That's why I haven't removed that bit completely. Please feel free to recommend a better way to say this if you've got an idea for a better way to explain this.