Docs caching update
Docs caching update

Summary:
I just noticed that I forgot to replace `ImageType` with `CacheChoice` in next paragraph:

> The pipeline will default to using the same cache for both and ImageType will be ignored.

I already had a [PR for first commit](https://github.com/facebook/fresco/pull/1781) which got approved, but this pr has both commits.
Closes https://github.com/facebook/fresco/pull/1782

Differential Revision: D5138764

Pulled By: lambdapioneer

fbshipit-source-id: 7f67bcaf67cdfb452e67dac275bffdb684558ab6

xjy2061/NovaImageLoader,xjy2061/NovaImageLoader,facebook/fresco,facebook/fresco,xjy2061/NovaImageLoader,xjy2061/NovaImageLoader,MaTriXy/fresco,MaTriXy/fresco,s1rius/fresco,facebook/fresco,MaTriXy/fresco,MaTriXy/fresco,s1rius/fresco,facebook/fresco,s1rius/fresco,xjy2061/NovaImageLoader,s1rius/fresco,MaTriXy/fresco,facebook/fresco,facebook/fresco