Using AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY as env vars is undocumented

[shapirus]

$subj.

These env variables work, but their use isn't documented properly. The reason they work is apparently because the AWS API library used by external-dns supports them. There is no clear description of it in the external-dns docs, one has to make guesses and dig into helm charts etc. where this may be tangentially mentioned.

see also: #1781