document DNS SRV record options - dotnet

(Modified description):

Please review and/or describe how to use DNS SRV in your SDK. The "generic" documentation was added in this commit: https://github.com/couchbase/docs-cb4/commit/9f6a083a9f55728dba0cb7c9736c309fbc421d71

Please document any additional steps needed for your SDK.

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For .NET, please follow the other SDKs (Java, C, Go) - even if you're only documenting how to do this with 3p tools - no need to go into excessive detail, but simply document how to do the Couchbase part, perhaps linking to a gist or similar for more in-depth explanation of the third party tool/lib/plugin.