Clarify role variable directory loading

[mogfrey]

Summary

Clarify how Ansible loads variables from role defaults and vars directories.

The previous wording could imply that any nested directory under defaults/ or vars/ is loaded automatically. The updated text explains that:

• Ansible automatically loads the main entry point.
• defaults/main and vars/main may be files or directories.
• Alternative files or directories must be selected explicitly with defaults_from or vars_from.

Fixes #2994.

Validation

• nox -s spelling
• nox -s "checkers(rstcheck)"
• nox --force-python 3.13 -s "checkers(docs-build)"

Scope

Documentation only. No runtime behaviour is changed.

[ansible-documentation-bot]

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