shortcut-ui: document uncomment/comment shortcuts

Merged Mathieu PATUREL requested to merge math2001/gnome-builder:add-comment-shortcut into master

I expected the keybinding to be dynamically generated, but it's just a .ui file right?

The string seem to be already translated (I quickly looked into a po file, and there is a line msgid: "Comment code" and the uncomment version too).

Fixes #1285 (closed)

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