Prefixed functions from grDevices and updated docs

[bvancil]

I noticed some discussion in issue #44 that functions in the package grDevices didn't have grDevices:: prefixes, which caused problems in checks for downstream package (mine included) that use extrafont. This update seems to play well with devtools::check().

I also found and fixed a typo in the roxygen2 docs and regenerated the documentation.

Please let me know if there are any other checks you'd like me to perform.