Document migration support for BYO schemas and schemaspaces

[kevin-bates]

Pull requests #2263 and #2282 provide different kinds of migration paths relative to instances of metadata schemas and schemaspaces. In these exercises, there are two forms of migration that surfaced and should be documented:

1. Implicit migration. This form of migration consists of migrating metadata instances within the same schemaspace. This will typically occur when a schema is deprecated in favor of another schema (or schemas) within the same schemaspace, or if changes are necessary within a current schema. No user intervention should be required for this schema. This form was introduced in Add support Runtime types #2263 and takes place via the metadata_class_name implementation.
2. Explicit migration. This form of migration consists of migrating metadata instances from one schemaspace to another and will typically occur when a schemaspace has been deprecated. With this form, the elyra-metadata tool can be used against the deprecated schemaspace and enlist the services of the underlying SchemasProviders that override the migrate() method.