Document behavior of empty ec2_sd_config region (#5711)

Document the behavior of an empty `ec2_sd_config` `region` setting. If this is
omitted or blank, the region is discovered from the instance metadata, if available.
If it is blank and instance region metadata is not available, an error will
result ("EC2 SD configuration requires a region").

Signed-off-by: Svend Sorensen <svend@svends.net>

docs/configuration/configuration.md

@@ -439,8 +439,8 @@ See below for the configuration options for EC2 discovery:
 ```yaml
 # The information to access the EC2 API.
 
-# The AWS Region.
-region: <string>
+# The AWS region. If blank, the region from the instance metadata is used.
+[ region: <string> ]
 
 # Custom endpoint to be used.
 [ endpoint: <string> ]