Document `$ion_literal` and work out edge cases.

[toddjonker]

This hasn't made it into the 1.1 spec somehow other than oblique mention in the intro's system symbol table section.

One open issue is how we deal with $ion_literal::$ion_symbol_table::{} and $ion_literal::$ion_1_0. If those work as expected for $ion_literal, it means 1.1 can encode a value that 1.0 cannot.

I think we need to pick one:

1. We require that both 1.0 and 1.1 represent the identical space of data model values. This means that conversion between the formats will always succeed. It also means that 1.1 must reject values that cannot be encoded by 1.0.
2. We say that data that cannot be encoded in 1.0 are indicative of defects of that format, and allow them to be encoded in 1.1