Improve Optional docs
Improve Optional docs

Summary:
TSIA

(Note: this ignores all push blocking failures!)

Reviewed By: praihan

Differential Revision: D38224492

fbshipit-source-id: 993e237fbc2968ffca0b425c62d098f0a9a54fa7

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly