[csharp][generichost] Updated docs (#21357) #882

Job	Run time
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	1m 53s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	2m 53s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	1m 53s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	2m 47s
	9m 26s

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