Add comments to clarify aws endpoints controllers

[enxebre]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, use fixes #<issue_number>(, fixes #<issue_number>, ...) format, where issue_number might be a GitHub issue, or a Jira story:
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• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.
• This change includes docs.
• This change includes unit tests.

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: enxebre

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[sjenning]

/lgtm
/retest-required

[openshift-ci-robot]

/retest-required

Remaining retests: 0 against base HEAD 1f70f40 and 2 for PR HEAD 4b04129 in total

[openshift-ci-robot]

/retest-required

Remaining retests: 0 against base HEAD bb64fb1 and 1 for PR HEAD 4b04129 in total

[openshift-ci]

@enxebre: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/capi-provider-agent-sanity	4b04129	link	false	/test capi-provider-agent-sanity
ci/prow/e2e-ibmcloud-iks	4b04129	link	false	/test e2e-ibmcloud-iks

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[enxebre]

/test e2e-aws