Add documentation for public types and modules

[popematt]

Documentation is missing from these types/modules (and possibly more). Even a one sentence description would be helpful.

• ion_rs::types::Int – even something as minimal as "An in memory representation of an Ion int"
• ion_rs::types::SymbolId
• ion_rs::element::builders
• ion_rs::result and all of its child modules
• ion_rs::element::reader::OwnedElementIterator

There are also more modules and types that are missing documentation, but I tried to focus on the ones that might remain public for the 1.0.0 release.