Docs String Freeze Exception: Update keyboard shortcuts

[Jeremy Bicha]

Design has requested window management keyboard shortcuts to be
changed to use Ctrl+Super instead of just Super. Full details are
available at https://bugs.launchpad.net/ayatana-design/+bug/969235

At the same time, I want to refer to the Windows key as the "Super"
key instead of as the "Meta" key. Meta is confusing as it also refers
to the Alt key. Since Unity's new keyboard shortcut overlay uses
"Super", anything else would be confusing.

I've submitted a merge with these changes at
https://code.launchpad.net/~jbicha/ubuntu-docs/lp969235-ctrl-super-keyboard-shortcuts/+merge/100546


Thanks,
Jeremy