Document logging for Key Vault clients

[chlowell]

The "Troubleshooting" section of Key Vault readmes demonstrate exception handling and claim that in the event of an error, "You will notice that additional information is logged, like the client request ID of the operation." The keys readme even shows an example of this log output. However, the example exception handling code doesn't log anything or produce that output, and the readmes don't describe how to obtain the output shown in the keys readme. Samples don't demonstrate logging configuration either.