Documentation Request Summary:

Mongos rejects all requests with readConcern level "snapshot" unless the startup parameter enableTestCommands is true. The default is false.

Global point in time reads isn't officially supported for 4.0 though, so I'm not sure if this qualifies as documentation changes or if we even need to document that they are blocked by the enableTestCommands flag.

Engineering Ticket Description:

The objective is to allow testing but prevent general use as multi-statement global snapshot reads are not supported in 4.0