CASSANDRA-21331: Improve incremental repair documentation clarity

[dybyte]

This PR improves the incremental repair documentation for CASSANDRA-21331.

It:

• explains why enabling incremental repair on an existing cluster can create significant anticompaction load
• clarifies how the first incremental repair differs from a full repair
• adds a direct anchor for the anticompaction reference in the compaction overview docs

Validation:

• ran ./.build/sh/ai-build

No CHANGES.txt entry is needed because this is a documentation-only change.

[Copilot]

Pull request overview

This documentation-only PR improves the operator guidance around enabling incremental repair on existing clusters by clarifying why the first run can be expensive (anticompaction load) and how the first incremental repair differs from a full repair. It also adds a direct anchor in the compaction overview docs so incremental-repair docs can link precisely to the anticompaction explanation.

Changes:

• Added an explicit #anticompaction anchor in the compaction overview documentation.
• Expanded incremental repair guidance to explain anticompaction-related load when enabling incremental repair on existing clusters.
• Clarified that the first incremental repair still compares the full unrepaired dataset, but (unlike full repair) updates incremental repair state for subsequent runs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
doc/modules/cassandra/pages/managing/operating/compaction/overview.adoc	Adds a dedicated #anticompaction anchor to enable direct cross-references.
doc/modules/cassandra/pages/managing/operating/auto_repair.adoc	Improves incremental repair documentation clarity and updates links to the new anticompaction anchor.

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