Improve functionality/documentation for `--generate-cli-skeleton`

The current behavior of `--generate-cli-skeleton` has caused some confusion (see https://github.com/aws/aws-cli/issues/5266 and https://github.com/aws/aws-cli/issues/6684).

Here in the code shows where a random value is chosen for strings: https://github.com/boto/botocore/blob/c3731390f7a98c35d13f766ba99e465c2ede9c78/botocore/utils.py#L942

I think that value should be set to whatever the default is, or the docs/user guide should clarify the current behavior: https://docs.aws.amazon.com/cli/latest/userguide/cli-usage-skeleton.html