Add section of modifying appGw settings in application-gateway-faq.yml

[JoeyC-Dev]

Proposed change: Add a section to tell users that "modifying application gateway settings directly" should not be recommended.

Supporting points: There are many users asking why their configuration is being changed w/o reason from time to time. And there is no document ATM telling users that the configuration is supposed to be modified from AGIC instead of AppGw directly or the consequence will be suffered.

Links to any related work item(s):

• Related PR: https://github.com/MicrosoftDocs/architecture-center/pull/4484