Clarifying documentation for CloudWatchConnection.list_metrics().

As far as I can tell, the CloudWatch API doesn't actually support the ability to specify multiple values for the same dimension.  In my experience, I only ever get results for one of the values listed.  For example, with:

```python
conn=cloudwatch.connect_to_region("...")
conn.list_metrics(dimensions={"InstanceId": ["i-xxxxxxxx", "i-yyyyyyyy", "i-zzzzzzzz"]})
```

I only every get results for i-zzzzzzzz.

This brings documentation for CloudWatchConnection.list_metrics() in line with http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/cli-mon-list-metrics.html, which demonstrates the same behavior.  i.e. `mon-list-metrics --namespace AWS/EC2 --dimensions "InstanceId=i-xxxxxxxx,InstanceId=i-yyyyyyyy,InstanceId=i-zzzzzzzz"` only returns results for i-zzzzzzzz.