?theme legend.position doesn't document option "none"

[gavinsimpson]

In ?theme, the information for legend.position states

the position of legends. ("left", "right", "bottom", "top", or two-element numeric vector)

The option "none" is missing from that list. The use of legend.position = "none" is documented in the examples, but it should be listed as an available option.

[hadley]

Could you please rebase/merge against master, re-document with the development version of roxygen2 (install_github("klutometis/roxygen) and resubmit?