Add missing docs

[ljedrz]

No description provided.

[shepmaster]

So... I deliberately left off a lot of these (not all) because they are pointless repetitions of the method name. Care to take a shot at convincing me what the value of this type of documentation adds?

/// An owned prefixed name.
struct OwnedPrefixedName

[ljedrz]

Docs like this aren't necessary, but potential crate users often evaluate crate quality based on the completeness of the documentation (I know I do when I don't recognize the crate's owner). Undocumented entries, especially in versions < 1.0 may come off as work in progress or feature-incomplete.