Basics: Add documentation note to radians function.

[ybakos]

Change 'Convert' to 'Represent', as the function does not perform conversion.

The documentation of the radians function has confused a few people - why have a function simply return its input? Per the discussion in https://github.com/elm/elm-lang.org/issues/713, add a note to the documentation explaining the value of the radians function.

References https://github.com/elm/elm-lang.org/issues/713.