[Docs] Document workaround when using private link endpoints with us-east-1

[robbavey]

As with the issue reported in the S3 input plugin in logstash-plugins/logstash-input-s3#234, when using private link endpoints with us-east-1, the plugin will fail to write to the specified bucket, producing the following error:

[2022-02-03T08:20:59,305][ERROR][logstash.outputs.s3      ][main] Failed to upload file (retried 3 times). {:exception=>Seahorse::Client::NetworkingError, :message=>"Failed to open TCP connection to mybucket.bucket.vpce-vpcid.s3.vpce.amazonaws.com:443 (initialize: name or service not known)"

This is due to a problem fixed in aws/aws-sdk-ruby#2483, but not backported to v2 of the AWS-SDK, where us-east-1 will be stripped from the url when using the IAD regional endpoint plugin (which is included by default).

This should be able to be circumvented by setting s3_us_east_1_regional_endpoint=regional in the aws config file.