Improve description, summary for XmlException

what's the Xml​Exception used for and where it can be thrown? literally I don't see any description in the MSDN doc

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#### Document Details

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* ID: 95744791-43ad-e667-64d2-abf09cef4c60
* Version Independent ID: 3e1c2554-ee91-1650-5ccb-94159ff20c64
* Content: [XmlException Constructor (System.Xml)](https://docs.microsoft.com/en-us/dotnet/api/system.xml.xmlexception.-ctor?view=netframework-4.7.2)
* Content Source: [xml/System.Xml/XmlException.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Xml/XmlException.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**