`Instant` support

Docs reflect that `Instant` is not a supported [graph property type](http://docs.janusgraph.org/latest/schema.html#_property_key_data_type) but is a supported [index backend type](http://docs.janusgraph.org/latest/search-predicates.html#mixeddatatypes). This is confusing and should be clarified.

Start with the linked graph property type and search-predicate documentation sections to compare how `Instant` is described. Clarify why it is unsupported as a graph property type but supported as an index backend type; the work is done when the two sections no longer appear contradictory.

Written by the indexing model from the issue text.