storage: clarify usage of Client and Client.Close
storage: clarify usage of Client and Client.Close

Document that Client should be reused rather than frequently
re-created (in anticipation of when it is on top of gRPC).

Also document that it is safe for use by multiple goroutines.

Document that Close needn't be called at exit.

Change-Id: I3cc57006834bdb0a0ba429daba3ba826098dfe9d
Reviewed-on: https://code-review.googlesource.com/7695
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go