Added missing documentation for a method and two params

I wanted to set the scaleMode for a Texture while using fromImage and I couldn't find that in the documentation on Textture: http://www.goodboydigital.com/pixijs/docs/classes/Texture.html

I added it to the fromImage and fromCanvas documentation. Later I also saw that BaseTexture.fromCanvas lacked documentation all together, so I added it too.