Mention kernelBlur on mirror documentation

[deltakosh]

http://doc.babylonjs.com/how_to/reflect#mirrors

Sample:
https://www.babylonjs-playground.com/#LVTTQX#1

[deltakosh]

MirrorTexture can support blurred rendering with either:

• adaptiveBlurKernel: setting this value to something else than 0 will blur the texture with a specified kernel (the bigger the blurrier). Value will be adapted to the viewport size
• blurKernel: same as adaptiveBlurKernel proeprty but the value is not adapted to viewport size

[BabylonJSGuide]

Done