Make Client.Close() more prominent in docs/examples

[gbrener]

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I encountered a "too many open files" error/panic when using the docker Golang client library as documented, and traced it back to this function: https://godoc.org/docker.io/go-docker#NewEnvClient . I solved the problem by calling the Close() method on the *Client object, but because this method is not being used in any of the examples, it took me a while to find it. Perhaps we can make Close() more prominent in the documentation somehow - and/or included in the existing examples - so that this doesn't bite more people in the future.