Document content.sha1hex field
Document content.sha1hex field

Summary: The content.sha1hex field has existed in Watchman since 2017 (version 4.9.0), but it isn't documented in Watchman's field list. Add it so it's easier to discover.

Reviewed By: simpkins

Differential Revision: D16435295

fbshipit-source-id: d94c0f54165b71b708d079dc7233e2f6199e21f3

facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman