docs: Improve documentation of `s3` source

[udf2457]

In the docs, s3 is mentioned as a permitted source.

However, unless I've missed it somewhere, the docs are silent on how to deal with s3 credentials ?

[prestist]

The documentation on S3 authentication and other detailed operational notes are found in Ignition's
operator documentation here: https://coreos.github.io/ignition/operator-notes/#aws-s3-access

The spec documentation in Butane (the config-*.md files) is auto-generated from Ignition's schema
definitions. Those docs focus more on field definitions and structure rather than runtime behavior.

Since Butane is a transpiler and doesn't interact with S3 itself (that happens at boot time by
Ignition), the credential information lives in Ignition's docs.

However, I can see how someone writing a Butane config with source: s3://... would naturally wonder how authentication works. We could potentially add a note or link in the generated docs pointing to Ignition's operator notes for
runtime details, wdyt?

[udf2457]

I think that's a fair compromise, sort of a little bridge between the silos.

[prestist]

Following up on the earlier discussion - we agreed that adding a link to Ignition's operator notes would help bridge the gap between the schema docs and runtime behavior.

The proper approach:

This needs to be fixed upstream in the Ignition repository first:

1. Add a link to the operator notes in the resource.source field description in Ignition's config/doc/ignition.yaml
2. The link should point to: https://coreos.github.io/ignition/operator-notes/#aws-s3-access
3. Once that's merged in Ignition, update Butane's vendored Ignition dependency
4. Run ./generate in Butane to regenerate the spec docs with the updated descriptions

Why upstream first:

Since Butane's spec documentation is auto-generated from Ignition's schema definitions (at vendor/github.com/coreos/ignition/v2/config/doc/ignition.yaml), the source of truth lives in the Ignition repo. Making the change there ensures it stays consistent across all tools that use Ignition.

For contributors:

This involves coordinating between two repositories (coreos/ignition and coreos/butane). You'll need to:

• Submit a PR to coreos/ignition first
• After that's merged, submit a follow-up PR to Butane to vendor the new version

Check out vendor/github.com/coreos/ignition/v2/config/doc/ignition.yaml lines 189 and 360 for examples of how other fields link to operator notes.