Undocumented fabric-ca-client flag: -d

The `fabric-ca-client` command supports an undocumented `-d` flag that sets the logging level to debug, overriding the `--loglevel` option.

This flag is used in tests and examples, but I couldn't find documentation for it.

It's defined (and marked as hidden) here: https://github.com/hyperledger/fabric-ca/blob/d3c7ae278391077ddfe03e9a80b94b00477173a8/lib/clientconfig.go#L44

This tripped me up because I was following example invocations of the command (which include the `-d` flag) and I was trying to reduce their verbosity via `--loglevel` but it didn't seem to work.
Only when I noticed and removed the mysterious flag did it start working as expected.