Doc - Describe `setup_hint_processor` using Rustdoc comments (three slashes: ///)

[Eikix]

In an effort of having fully documented code, we need the way we add custom hints (mocking calls, skipping instructions, expecting revert) to be documented.

In src/cli/commands/test, please proceed to comment above the fn setup_hint_processor declaration in an equivalent way as pub fn compile is documented.

[jdembdr]

Ready to go.

[RuneRogue]

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