Add missing docstrings and improve clarity in doc_controls module

[Krishnamohan33]

This PR improves documentation clarity in the doc_controls module.

Changes include:

• Added missing docstrings for helper functions (is_deprecated, should_doc_private)
• Improved clarity of existing docstrings (get_custom_page_builder_cls)
• Ensured consistent docstring formatting

No functional changes were made. These updates improve readability and make the code easier to understand for new contributors.

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[Krishnamohan33]

I signed it