This PR was squashed before being merged into the 2.2.x-dev branch (closes #1399).

Discussion
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Added argument description to twig asset() method

I have added a description of the arguments for the asset() method. I didn't find them obvious when I first used the AssetServiceProvider, although it makes sense in hindsight. The description would have saved me some time getting the paths right

Commits
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2f5d870f Added argument description to twig asset() method