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

[Eikix]

In an effort of having fully documented code, we need hooks to be documented.

Please proceed to comment above the pub fn pre_step_instruction declaration in an equivalent way as pub fn compile is documented.

[ptisserand]

Hi, can I work on this issue?

[Eikix]

Sure thing! 🔥