Update to std.stdio documentation for File.readln variants

Cf the discussion at http://forum.dlang.org/thread/georhjhecdjfvmcbxbpr@forum.dlang.org. Doesn't fix the issues described for File.readln(buf) not reusing the passed-in buffer in the cases of type dchar or wchar. Merely updates the docs to fix an issue (docs attached to the buf = File.readln() version were simply wrong; too many dittoes!) and expand the example for File.readln(buf) to make usage more clear and, hopefully, help others avoid making the mistake I did.