Improve doc wrt allow_upscale

[chrisjohnson00]

Can you add a quick note for what allow_upscale does when set, with an example of how to set it? Also, what filter types does this apply to (thumbnail vs relative_resize, etc)?
I guess by the name that if you set allow_upscale = true, then if an image is smaller than the specified size it will not be made larger.

[chrisjohnson00]

I've read through the thumbnail filter implementation and think I can update the docs for this, will create a pull when I have time.
For anyone who reads this, allow_upscale is only supported in thumbnail, not relative_resize.