Update docs for delegated conditions

🤔 Description

In gitlab-org/gitlab!90963 (comment 1067855804), we needed to reference conditions on a delegated policy. It turns out that this is supported by this gem as long as we use named delegates.

This was not clearly documented, even though there are some specs for this support.

This MR updates the documentation by adding a section for delegated conditions with the Delegates section.

In addition, this MR updates the specs so that delegated conditions using bare words are covered.

⛓ Related Issues

n / a

⤴ Suggested version bump

Being a documentation update, I don't think we need a version bump. This change can wait the next bump triggered by code changes.

• [-] Major (backwards incompatible changes)
• [-] Minor (backwards compatible changes)
• [-] Patch (API compatible changes)

Checklist

I don't think that a documentation update warrants a CHANGELOG.md update.

• Tests have been added or updated to cover the documentation changes.
• This does not change the API to consume this library, or a suggested version bump has been provided
• [-] The CHANGELOG.md has been updated
• [-] No new runtime dependencies have been introduced