Start adding zero-copy docs

[CraftSpider]

Adds documentation to all zero_copy items And one or two minor changes I couldn't resist while documenting

[CraftSpider]

Unrelated note: After this PR, definitely think it might be a good idea to pull Container, Seq, etc into their own module, to track these common 'container helpers'.

[CraftSpider]

Currently recover_with fails, and two docs are TODO. On the other hand, I think it may be worth to merge now and fix those things later, since this change is very prone to bitrot

[zesterer]

Do you think it's ready for merge?

[CraftSpider]

I'm going to add at least basic documentation to the two TODO ones quick, then I think so

[CraftSpider]

I'd call this PR ready to merge - there are one or two rough spots, but overall I think this improves the current state of zero-copy documentation.

[zesterer]

Thanks! This is a big step forward :)