Please document what this method checks

The fact that this method gets a double and returns a bool is obvious from the signature. But whan does IsNormal actually check?

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#### Document Details

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* ID: 51bd4b74-5154-902f-a99c-74ae79628081
* Version Independent ID: e943c657-b075-3abe-7319-6f58eace2dbb
* Content: [Double.IsNormal(Double) Method (System)](https://docs.microsoft.com/en-us/dotnet/api/system.double.isnormal?view=netcore-2.1)
* Content Source: [xml/System/Double.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System/Double.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**