Docs for dataframe programmatic selections

[MathCatsAnd]

Describe your changes

Update docs for programmatic selections for dataframes and include an example.

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[github-actions]

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[Copilot]

Pull request overview

Updates Streamlit’s st.dataframe documentation (docstrings in arrow.py) to better describe programmatic selection via Session State and to add a concrete example for setting selections.

Changes:

• Adjusted DataframeSelectionState docstring text around programmatic selection support and expanded to multiple examples.
• Added a new example demonstrating programmatic selection updates via st.session_state for st.dataframe.
• Simplified/clarified the key parameter docs for reading/updating selection state via Session State.

[lukasmasuch]

LGTM 👍