Some flags are missing documentation

[gibson042]

Bug Description

f811a0e renamed -fenable-tdz to -Xenable-tdz, but --help does not appear to document the flag, resulting in tc39/eshost#125 .

Hermes version: 0.11.0
React Native version (if any): n/a
OS version (if any): n/a
Platform (most likely one of arm64-v8a, armeabi-v7a, x86, x86_64): n/a

Steps To Reproduce

$ hermes --help | grep tdz

The Expected Behavior

At least one line including "Xenable-tdz".

[tmikov]

Hi,
-Xenable-tdz is unsupported and should not be used, it can be seen with "-help-hidden".