add "none" to documentation of theme() for parameter legend.position

[krlmlr]

Reissue of #831.

[hadley]

• Motivate the change in one paragraph, and include if affects code,
  include it in NEWS. In parenthesis, reference your github user name and
  the issue, (@hadley, #1234)
• Check pull request only includes relevant changes.
• Use the official style.
• Update documentation and re-run roxygen2

You don't need to change the NEWS here, but generally I think it's only worthwhile to list major documentation changes.

[lock]

This old issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with reprex) and link to this issue. https://reprex.tidyverse.org/