[Docs] assume_role support needs to be documented across multiple plugins

[robbavey]

logstash-plugins/logstash-mixin-aws#27 was introduced earlier this year, which gave the ability for AWS plugins that use the AWS mixin library to support assuming roles.

This is however, not documented anywhere, and there are some pitfalls that users can easily fall into when attempting to use the feature. The feature should be documented in each of the plugins that support it:

• logstash-input-s3
• logstash-input-sqs
• logstash-output-cloudwatch
• logstash-output-sns
• logstash-output-s3

It is also used in this plugin, which is currently undocumented (issue)

• logstash-input-s3sqs

cc @karenzone (If you would rather move this issue elsewhere for better visibility, be my guest)

[karenzone]

Note to self:
AWS Identity and Access Management (IAM) is a web service that helps you securely control access to AWS resources. (https://docs.aws.amazon.com/IAM/latest/UserGuide/introduction.html)