message: update doc comments
diff: -//! Decoding of BMP Images
  +//! Decoding and Encoding of BMP Images
  //!
  -//! A decoder for BMP (Windows Bitmap) images
  +//! A decoder and encoder for BMP (Windows Bitmap) images
  //!
  //! # Related Links
  //! * https://msdn.microsoft.com/en-us/library/windows/desktop/dd183375%28v=vs.85%29.aspx
  