Add a doc string for Set-TestTimeout.
Add a doc string for Set-TestTimeout.

jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,jsimonweb/csharp-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples,GoogleCloudPlatform/dotnet-docs-samples