Add note about `appgw.environment`

[timothymeyers]

appgw.environment is a required parameter for deploying into Sovereign Clouds, but it is not documented on this page. I had to go into the AGIC source code repo and find a troubleshooting doc that identified it. Adding it to the values list and adding a note so that it gets attention.

[prmerger-automator[bot]]

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[opbld31]

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[Jak-MS]

@greg-lindsay

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[greg-lindsay]

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