[docs] Document keystore eligible settings on S3 repository plugin and AWS discovery plugin

[bczifra]

The AWS discovery and S3 repository pages make no reference to the fact that the AWS credentials can be retrieved from the keystore, despite the fact that the secure settings documentation indicates:See documentation for each setting to see if it is supported as part of the keystore.

I can't seem to find any setting at all that includes any keystore-related information as part of its document. Right now I think this is pretty confusing for our users. Additionally, @rjernst indicated that

setting discovery.ec2.access_key in elasticsearch.yml should not work. This setting should only be allowed in the keystore

so, if I understand that correctly, we shouldn't be showing that this can be set in the elasticsearch.yml file.

Also, based on this post I think the cloud.aws.* settings need to be updated everywhere to instead be discovery.ec2.*, and cloud.aws.region should be discovery.ec2.endpoint.

[dadoonet]

@rjernst knows it. :) I'm marking that as a blocker for 6.0 final.
Feel free to remove.

[bczifra]

@dadoonet awesome. i didn't quite understand feel free to remove. remove what?

[dadoonet]

I meant. Feel free to remove the blocker label.
Sorry :)

[rjernst]

This was addressed with #26033 and #26065