docs: add section in uf article on discoverability

[petar-i-todorov]

refs https://github.com/telerik/reporting/issues/5448

[Copilot]

Pull Request Overview

This PR adds documentation about user function discovery behavior in Telerik Reporting, specifically addressing when and how user functions are automatically loaded by the reporting engine.

Key Changes:

• Added a new "User Function Discovery" section explaining the security-focused discovery mechanism
• Documented that user functions are only discovered from explicitly configured assemblies
• Explained why functions from the executing assembly are not automatically discovered

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[DimitarNikolovv06]

looks good