Need to document that Elements need to have NodeKind IRI and all other objects are blank

This came up in the implementers call.

Based on prior discussions, we agreed that `Elements` must be an IRI so they can be referenced across different serializations and be guaranteed unique.  Non-Element objects (e.g. `CreationInfo`) should always be included in the same serialization - we could enforce this by requiring blank nodes.

Reference https://github.com/spdx/spec-parser/pull/107
