Document StatusValue better and add per-resource restrictions

This generalizes the "LINKED can't be anywhere" idea into more
targeted restrictions.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146158204

javatests/google/registry/rde/DomainResourceToXjcConverterTest.java

@@ -408,9 +408,7 @@ public class DomainResourceToXjcConverterTest {
             .setLastEppUpdateClientId("CeilingCat")
             .setLastEppUpdateTime(DateTime.parse("1920-01-01T00:00:00Z"))
             .setRepoId(repoId)
-            .setStatusValues(ImmutableSet.of(
-                StatusValue.OK,
-                StatusValue.PENDING_UPDATE))
+            .setStatusValues(ImmutableSet.of(StatusValue.OK))
             .build());
   }
 }