Update docs about LithoAssertions
Update docs about LithoAssertions

Summary: Per title

Reviewed By: mihaelao

Differential Revision: D5547079

fbshipit-source-id: 33a27ba85cffb90db33a0c7f0dbd47247bd0031f

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho