WriteConcern argument description is incorrect for the moveChunk command

Description

Here we say this:

writeConcern document Optional. A document that expresses the write concern that the _secondaryThrottle will use to wait for secondaries during the chunk migration. Any specified writeConcern implies _secondaryThrottle and will take precedent over a contradictory _secondaryThrottle setting.

This implies that you can specify any combination of writeConcern and _secondaryThrottle arguments, and that should work. However, the server will only accept the command with writeConcern if _secondaryThrottle is not set to false:

> db.runCommand({moveChunk:"test.docs", find: {x:60}, to: "shard02", _secondaryThrottle:false, writeConcern:{"w":3}})

{

    "code" : 12,

    "ok" : 0,

    "errmsg" : "Cannot specify write concern when secondaryThrottle is not set"

}

Perhaps the following line needs to be removed:

Any specified writeConcern implies _secondaryThrottle and will take precedent over a contradictory _secondaryThrottle setting.

Scope of changes (files that need work and how much)

Impact to other docs outside of this product

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