Missing UseAppPoolCredentials from IIS Config Reference for WindowsAuth

[Rich-Lang]

This is one or the more important config settings for customers that use Windows Auth in WebFarms. Its description is missing from the config reference.

https://docs.microsoft.com/en-us/iis/configuration/system.webserver/security/authentication/windowsauthentication/

<attribute name="useAppPoolCredentials" type="bool" defaultValue="false" />