Unchecked indexing/slicing methods should point towards their safe counterparts

[jonas-schievink]

The following unsafe unchecked indexing / slicing methods don't refer to the safe alternatives at all, misleading newbies into thinking that they are the preferred way of performing that operation. There should be a big disclaimer stating what their checked counterparts are and that they are preferred.

• str::slice_unchecked / str::slice_mut_unchecked
• [T]::get_unchecked / [T]::get_unchecked_mut

[Mark-Simulacrum]

cc @rust-lang/docs, this feels like an E-easy issue to me, tagging as such -- we should add the links to the unchecked method documentations pointing to safe indexing (get, probably, where applicable as well as noting indexing with the Index trait). Does that sound good?

[GuillaumeGomez]

@Mark-Simulacrum: Yep, all good.

[citizen428]

If no-one picks it up until then I'll try to get this done over the weekend.

[GuillaumeGomez]

@citizen428: Sure, go ahead.