add -F information to description

[mlees0209]

It looks like the documentation has not been updated along with the addition of -F functionality -- I am proposing a small change to correct that.

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[PaulWessel]

Please instead say. "Finally, use option |-F| to set all nodes inside or outside the specified polygon to NaN.

[PaulWessel]

I have done this but a bit differently Closing as finished,