Publish migration guidance for downstream users affected by explicit feature management

[nradakovic]

What

Publish migration guidance for downstream users who may rely on behavior previously provided implicitly by Bazel legacy features.

The goal is to help consumers understand the impact of the explicit-feature model and what actions may be required in their workspaces.

Acceptance Criteria (DoD)

• A migration document or release-note style guide exists for downstream users.
• The documentation explains the impact of enabling no_legacy_features.
• The documentation identifies behaviors that are no longer implicitly available.
• The documentation explains how consumers can restore supported behavior explicitly when needed.
• The guidance is clear enough to support downstream adoption without source-code archaeology.

How

• Summarize the behavioral differences introduced by the explicit-feature model.
• Identify which previously implicit behaviors remain supported explicitly and which do not.
• Provide examples showing how consumers can register additional feature definitions via extra_known_features or extra_enabled_features.
• Include common migration scenarios where concrete examples would reduce support burden.
• Publish the migration guidance alongside the feature documentation and extension API material.

Out of Scope

• One-off migration support for individual downstream repositories.
• Feature implementation work.
• CI or test infrastructure changes.