Add examples for "make help" command

…mpl, this message will be displayed instead

Add more comments for the global understanding

…pnkfelix

Add rustc_on_unimplemented for Index implementation on slice

Reopening of rust-lang#31071.

It also extends the possibility of `#[rustc_on_unimplemented]` by providing a small type filter in order to find the ones which corresponds the most.

r? @pnkfelix

Improve help for make command

r? @steveklabnik

…veklabnik

trivial tweaks to documentation (book)

These are small things I found while reading through The Book. The `<hash>` and `panic!` lines are simply to improve readability, while I believe the proceeding/following distinction is a bug (but might be a English dialect distinction?).

I've read `rust/CONTRIBUTING`, i'm not sure if there is anything I need to do other than submit this PR.

r? @steveklabnik