Update API docs and conventions for fail -> panic

[brson]

Many of the in-tree API docs still have #Failure sections. The new convention appears to be #Panics.

[subhashb]

@steveklabnik I see you handled most of it in #17894 😄
Shall I take a stab at this cleanup?

[steveklabnik]

@subhashb sure, if you'd like to, feel free :)

[Gankra]

@subhashb Note that I fixed a few in collections in my reform branch. Hopefully will land today.

[subhashb]

@gankro Noted. Will skip around those files while your PR gets merged.