Add the api-version to the doc comment for the Client type

[richardpark-msft]

In a customer thread it came up that it's not obvious which version of the API is being supported by a package. We do document this in some places (for instance, in the operations), but not at the constructor func or Client level.

I'm wondering if we have some spots where we could do this that would make sense. In Go, at least, we seem to specifically support a single API version when we generate a package, so a Client level comment should be safe to add.

This only partly solves things, since the generated types are not always exposed directly, but it would be a step in the right direction, and, I think, directly exposing generated types is something we're moving towards.

CC: @jhendrixMSFT