Update doc for Incremental Mount
Update doc for Incremental Mount

Summary: Elaborated on Incremental Mount, corrected code sample

Reviewed By: andrewpmsmith

Differential Revision: D33186875

fbshipit-source-id: 9e477fe1e02aba7f4a85e370e475fe63aea8300b

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