Clarify the drawbacks of unsafe functions

Hi, I'm opening this issue regarding the documentation of the following functions:

---
Nowhere in the module documentation or in those functions it is said clearly the fact that:
* Given an empty vector, `head` will raise a runtime error;
* `unsafe` functions do not lead to RTS exceptions (like they may do in other container-like APIs), but will **segfault** the program. Beginners and more generally people who did not experience this behaviour first-hand are in no way warned of the implications of using `unsafe` functions (who are then closer than Rust’s `unsafe` than the commonly accepted definition of safety in the Haskell ecosystem).

I personally think it is dangerous to leave such gaps in the documentation and wanted to discuss this with the maintainers before opening a PR to rectify this.

cc @cartazio