docs: fix _merge_super_component_pipelines docstring (#12656)

[Taranum01]

Summary

Fixes #12656.

The docstring on PipelineBase._merge_super_component_pipelines described a function that does not exist in the source: it referenced parameters (parent_pipeline, child_components, merge_strategy, warn_on_conflict) and a return shape that don't match the actual implementation. This PR rewrites the docstring to match the real signature and behavior.

No code changes; documentation only.

Changes

• haystack/core/pipeline/base.py (around _merge_super_component_pipelines)
  • Rewrote the docstring to describe the actual parameters, return value, and side effects.
  • Aligned wording with the implementation (component-name collisions, graph mutation, warning emission).

Tests

No new tests added, per the maintainer note on the issue.

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