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

[Eikix]

In an effort of having fully documented code, we need the way we run tests on a single Cairo entry point to be documented.

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

[gdnathan]

I'd like to contribute on this issue