HBASE-19773 Documenting how to get instance of type RawCellBuilder

hbase-common/src/main/java/org/apache/hadoop/hbase/RawCellBuilder.java

@@ -23,6 +23,7 @@ import org.apache.yetus.audience.InterfaceAudience;
 
 /**
  * Allows creating a cell with {@link Tag}
+ * An instance of this type can be acquired using RegionCoprocessorEnvironment#getCellBuilder.
  */
 @InterfaceAudience.LimitedPrivate(HBaseInterfaceAudience.COPROC)
 public interface RawCellBuilder extends CellBuilder {

hbase-common/src/main/java/org/apache/hadoop/hbase/RawCellBuilderFactory.java

@@ -23,6 +23,8 @@ import org.apache.yetus.audience.InterfaceAudience;
 /**
  * Factory for creating cells for CPs. It does deep_copy {@link CellBuilderType#DEEP_COPY} while
  * creating cells.
+ * This is private because coprocessors should get an instance of type {@link RawCellBuilder}
+ * using RegionCoprocessorEnvironment#getCellBuilder.
  */
 @InterfaceAudience.Private
 public final class RawCellBuilderFactory {