Clarify the use of the visibilityProperty and timestampProperty schema fields

[GCHQDeveloper314]

The "Optional Element fields" section of the Schema docs states that Edges and Entities can optionally have visibilityProperty and timestampProperty fields.

However, the page doesn't give examples of how these should be used in a Schema. It's not clear if these properties can be specified for Edges and Entities. They are not used in this way in example schemas for Gaffer tests. These fields are instead used at a global level within a schema, with the timestampProperty field being nested inside config (also mentioned in the deprecations):

{
  "entities": {},
  "edges": {},
  "visibilityProperty": "visibility",
  "config": {
    "timestampProperty": "timestamp"
  }
}

Further detail should be added to this page so it's clear how the visibilityProperty and timestampProperty fields can be used. It might be possible to use these properties at the level of specific Edges and Entities as well as globally - this needs to be confirmed.