Add missing attribute documentation

[tobz1000]

Added docs for missing Container attributes. These were mostly done by inspecting serde_derive source; not by testing the functionality. Hopefully the descriptions are accurate.

Added docs for attributes which can specify different options for Serialize and Deserialize (fixes #77). I've also grouped these different variants of each attribute together, although I'm not sure how gitbook will render them. Let me know if this styling/grouping could be improved.

[dtolnay]

Thanks!

[dtolnay]

I took the field_identifier and variant_identifier sections back out because as the explanations even say, those attributes are probably not useful for the purposes that a reader would probably be on this page. They would be better explained on a page about handwriting Deserialize impls for a struct or enum.