doc: JSONPointable and JSONSetable are ambiguous

[dolmen]

The documentation for interfaces JSONPointable and JSONSetable doesn't mention how the argument to methods JSONLookup and JSONSet must be interpreted. As the pointable/setable value may be not just flat be instead structured, the documentation should tell if:

• the value is just the raw (decoded) name of a property or an array index (JSONLookup must only go one single level deeper)
• the value is itself the tail of a JSON Pointer
• something else

This is important for implementation to choose how to interpret characters / and ~.

Example: https://play.golang.org/p/81-pIl7Vjwc

type X struct {
     Y struct {
        Z string
     }
}