docs: add note about aws creds

[bmorelli25]

Backstory

@bmorelli25:

The AWS module documentation shows up in the metricbeat, filebeat, and heartbeat docs, but always refers to metricbeat config. See the filebeat docs for an example: https://www.elastic.co/guide/en/beats/filebeat/master/filebeat-module-aws.html#_supported_formats_2.

@kaiyan-sheng:

Yes this is by design. Because for all AWS related modules, no matter it's in Metricbeat, Filebeat or Functionbeat, it's all using the same credential options for authentication with AWS. Maybe under the Supported Formats section, we should remove the metricbeat.modules: line so it's less confusing? Thank you!

Proposed solution

@kaiyan-sheng, how do you feel about this solution? There are a lot of references to Metricbeat, so I'm not sure I want to remove the metricbeat.modules line. This PR adds a note at the top of the instructions that says Metricbeat is used for examples, but that the examples are applicable to any beat.

Related

For #24767.

[elasticmachine]

Pinging @elastic/obs-docs (Team:Docs)

[elasticmachine]

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• Start Time: 2021-08-17T19:53:11.699+0000

• Duration: 23 min 24 sec

• Commit: 2f3c1ad

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[kaiyan-sheng]

LGTM! Thank you!!