Panic condition documentation consistency

[numberten]

I noticed that the formatting of panic conditions in the documentation of a lot of functions seems to be inconsistent. Specifically it varies between simple warnings in the description of the function and 'Panics' subheaders with the panic condition listed below.

A good example being the difference between split_at and chunks_mut here.

I like that headers make it very explicit (visually) where the panic conditions in a function call are, but they can also lead to confusion since the same header used multiple times on a given page makes any links to them (via url#Header) direct to the topmost 'Panic' header.

[steveklabnik]

In general, they should all be with the header, yes. At this point, since we're still doing a lot of work on the std docs, I'm doing this as I go, rather than trying to do a whole audit of every function. So I've submitted a PR for split_at, and if you can enumerate any other places, I'm happy to knock them out.