SCHEMA-276. Clarify schema hash/id system in documentation.

From the mailing list:

| From the docs one would get the impression that the json avro schema
| is stored along with each value (cfr. the managing data page: "field
| value contains an the JSON representation of an Avro schema"), but
| from closer inspection I get that it only stores an id or hash of
| the schema, and schema's are retrieved from a meta table based on
| this id. It might be good to clarify this in the docs.